Xylazine and fentanyl are commonly used in combinations for immobilisation of wild antelope. In order to ascertain the effects of the combination of these drugs on certain metabolites and hormones in ruminants, blood was sampled from 8 karakul sheep (4 experimental and 4 control) and one tame blesbok (Damaliscus dorcas phillipsii) for 30 min before and after immobilisation. The samples were assayed for glucose, free fatty acids, insulin, thyroxine, triidothyronine, progesterone and oestrogen. Significant changes, after the administration of xylazine and fentanyl, were recorded in circulating concentrations of glucose, which increased, and free fatty acids and insulin, which decreased. The other hormones tested were not affected within the sampling period. It is suggested that the combination of xylazine and fentanyl may act directly on pancreatic Beta cells to inhibit the secretion of insulin, which will consequently affect circulating concentrations of glucose and free fatty acids.