Three cows within their 26th week of lactation (average milk yield: 15.7 kg/day) were used to determine the effects of starvation on 19 free amino acids (AA) of plasma, uremia and glycemia. For three successive days, blood samples were collected at hourly intervals from 09.00 am to 03.00 pm. From 1 month before up to the 1st day of the sampling period, the animals were overfed in two equal meals (06.15 am and 03.30 pm) with a hay-based diet. On the 2nd and the 3rd day, only water was available. On the 1st day of fasting, muscle protein catabolism increased as suggested by an increase (P < 0.05) in mean plasma levels of 3-methylhistidine (MeHis, index of muscle protein catabolism in cattle), Asp, Gly, His, Lys and Ser. On the same day, mean plasma concentrations of Glu and Pro decreased (P < 0.05), possibly through an enhanced utilization of those AA for gluconeogenesis and/or energy needs as suggested by the maintenance of glycemia and the increase (P < 0.05) in uremia. On the 2nd day of fasting, further changes of metabolic parameters occurred: mean plasma levels of MeHis, urea, Arg and Leu still increased (P < 0.05); those of Asn, Asp, Glu, Met and Tyr decreased (P < 0.05); plasma Gly, His, Lys and Ser plateaued at high and plasma Pro at low levels. Unlike results reported in dry cow, plasma Ala, Gln, Ile and Val did not increase during the fasting period. Furthermore, despite an enhanced protein mobilization, glycemia and milk yield dropped (P < 0.05) on the 2nd day of fasting. The fall in plasma glucose probably resulted from a continuous glucose uptake by the mammary gland for milk lactose synthesis which was not entirely compensated by gluconeogenesis. Within-day variations in plasma metabolites were observed, but these variations were generally lower during fasting than during feeding, especially for plasma glucose which showed variation with time of day during both fasting days. It was concluded that, as in dry cows, a 2-day fasting enhances muscle proteolysis in lactating cows. Contrary to dry cows, however, gluconeogenesis mechanism which attempts to maintain glucose homeostasis becomes insufficient in lactating cows to avoid a drop in glycemia from the 2nd day of fasting. The decrease in milk yield might be, at least partly, the result of that drop in glycemia. Finally, the time course from the last meal is a factor affecting within-day variations of plasma metabolites.Trois vaches en 26 e semaine de lactation ont ete utilisees pour determiner les effets de la mise a jeun sur l'aminoacidemie libre, l'uremie et la glycemie. Les echantillons de sang ont ete pre-leves d'heure en heure pendant 3 j successifs, de 9 h a 15 h. Jusqu'au 1 e r jour de mesures inclus, les animaux etaient alimentes (6 h 15 et 15 h 30) au-dela de leurs besoins. Les 2 e et 3 e jours, seule l'eau de boisson etait disponible. Au 1 e r jour de jeune, une augmentation du catabolisme des proteines musculaires, appreciee par une augmentation (p < 0,05) de la teneur plasmatique moyenne en 3-methylhistidine (MeHis), a ete observee. En outre, un accroissement (p < 0,05) des concentrations moyennes en uree, Asp, Gly, His, Lys et Ser, une diminution (p < 0,05) de celles en Glu et Pro ainsi que le maintien de la glycemie et de la production laitiere ont ete observes. Le 2 e jour de jeune s'est caracterise par des modifications d'un plus grand nombre des parametres etudies, notamment par un accroissement (p < 0,05) des niveaux moyens en MeHis, uree, Arg et Leu, par une diminution (p < 0,05) de ceux en Asn, Asp, Glu, Met et Tyr, et par un maintien a des valeurs hautes de la Gly, His, Lys et Ser, et a des valeurs basses de la Pro. Malgre cette accentuation apparente de la mobilisation proteique, la glycemie et la production laitiere ont chute au cours du 2 e jour de jeune. Par ailleurs, des variations intrajournalieres ont ete observees mais, comparativement a la situation nourrie, elles ont ete generalement faibles pendant le jeune pour tous les parametres metaboliques etudies, et particulierement pour la glycemie. Il a ete conclu que la vache en lactation mobilise rapidement ses proteines musculaires au cours du jeune. Cependant, le mecanisme de la neoglucogenese ne permet d'assurer l'homeostasie glucidique qu'au cours des 24 premieres heures de jeune. La chute de la glycemie serait, du moins en partie, responsable de la chute de la production laitiere au 2 e jour de jeune. Finalement, le temps ecoule depuis le dernier repas est un facteur affectant les variations intra-journalieres de l'aminoacidemie, de l'uremie et de la glycemie.