Circadian variations of plasma melatonin and prolactin concentrations were determined during breeding as well as non-breeding seasons in yak. Blood samples (5ml) were collected during different phases of estrous cycle, viz. early (0–6 days), mid (7–12 days) and late luteal (13–19 days) at 2h interval for 24h from eight yaks during one breeding month (November); the same yaks were bled at 2h interval during one non-breeding month (February) for 24h. Plasma melatonin concentrations rose sharply (P<0.01) after sunset to record peak concentrations between midnight and 2a.m. declining sharply thereafter in both breeding as well as non-breeding seasons. Basal melatonin concentrations were recorded between 0600 and 1600h. Stage of luteal phase did not influence the diurnal hormone change (P<0.01). In the breeding season, mean plasma prolactin concentrations displayed circadian variations with maximum value at 0400h (41.22±1.5ng/ml) and minimum at 1400h (12.0±4.02ng/ml). In the non-breeding season plasma prolactin concentrations showed circadian variation with maximum value at 0000h (59.9±10.5ng/ml) and minimum at 1200h (32.13±3.2ng/ml). A positive correlation in breeding (r=0.75) and in non-breeding season (r=0.65) between circadian changes in mean plasma prolactin and melatonin concentrations were seen. Circadian changes of mean plasma melatonin concentrations during breeding and non-breeding seasons were not different (P>0.05). However, mean plasma prolactin concentrations were found to be higher (P<0.01) in the non-breeding season. Three conclusions were drawn from the study: (i) melatonin and prolactin concentrations followed a circadian pattern of secretion (ii) melatonin and prolactin secretion may be closely interrelated and (iii) higher prolactin concentrations during the non-breeding season could be due to nutritional and environmental stress and hence might be contributing to lack of cyclicity.