Eight‐month‐old Jersey bull calves given ivermectin intravenously or subcutaneously showed signs of depression, ataxia, difficulty in breathing, tachycardia, salivation, diarrhoea, miosis, and an increase in pseudocholinesterase activity. The clinical signs were severe in calves given the drug intravenously. The findings suggest that the cholinergic nervous system may be involved in some of the adverse effects of ivermectin observed in calves.