The experiment was carried out to study the effect of animal species (Mediterranean buffalo, Bubalus bubalis, bulls; Friesian bulls and Delle Langhe rams) and four diets differing for forage:concentrate ratios (A = 12.5:87.5, B = 25.0:75.0, C = 37.5:62.5, D = 50.0:50.0) on passage rate of solid particles, measured by faecal Cr, and fluids measured by either faecal Co or disappearance of Co from the rumen. Significant differences were found, relatively to solid passage rate from reticulo-rumen (k 1 ) estimated by a multicompartmental model, between species (k 1 = 2.46 vs. 2.99 and 2.84% h - 1 for buffalo, cattle and sheep, respectively) and between diets (k 1 = 3.15, 2.71, 2.71 and 2.48% h - 1 for A, B, C and D, respectively). No significant difference was recorded, relatively to fluid passage rate from the rumen k 1 (faecal Co), estimated by Gamma4 model, between the three species (k 1 = 6.98, 6.71 and 6.92 for buffalo, cattle and sheep, respectively), while significant differences were found between diets (k 1 = 7.58, 6.92, 6.42, and 6.25% h - 1 for A, B, C and D, respectively). No significant difference was found, relatively to fluid passage rate (Co disappearance from the rumen), estimated by a negative exponential equation, between species (k 1 = 6.60, 6.38 and 6.74 for buffalo, cattle and sheep, respectively) or diets (k 1 = 7.08, 6.86, 6.33, and 6.02% h - 1 for A, B, C and D, respectively).