Ruckebusch, Y.&Allal, C. Depression of reticulo‐ruminal motor functions through the stimulation of α2‐adrenoceptors.J. vet. Pharmacol. Therap.10, 1‐10.Inhibition of the cyclical contractions of the reticulum and the rumen by detomidine (10–40 (μg/kg, i.v.), xylazine (20–80 μg/kg, i.v.) and clonidine (2.5–10 μg/kg, i.v.) were compared in sheep and cattle housed individually in box stalls. Two (X2‐adrenergic receptor blocking agents, tolazoline and yohimbine, were administered intravenously for prevention (0.1‐0.4 mg/kg) or reversal (0.4‐1.2 mg/kg) of these effects. Continuous recording of the reticulo‐ruminal contractions and measurement of the volume of ruminal gas eliminated through the upper respiratory tract indicated that the three a2‐agonists inhibited the primary ruminal contractions associated with the reticular contractions. The occurrence of secondary ruminal contractions was also blocked in sheep, but only suppressed in cattle. The inhibition of reticulo‐ruminal contractions was prevented or reversed competitively by the two ct2‐blocking agents, suggesting an ci2‐adrenoceptor mediation of the inhibition of cyclical motor activity of the reticulo‐rumen. In contrast with tolazoline, yohimbine was unable to alleviate the accumulation of gas resulting from inhibition of t