Abstract: An injectable ivermectin composition having a programmable release rate and providing multiple concentration peaks of active ivermectin to produce a sequence of ivermectin release pulses into the blood of cattle and horses. The composition comprises a solution of 0.2%-10% w/w of ivermectin in a solvent selected from propyleneglycol and a mixture of glycerile caprylate, caproate and caprate, such as glycerides of caproic, caprilic and capric acids in equal parts. The solution is used as a vehicle for suspending microspheres 100 nm-200 .mu.m in diameter of a degradable polymer containing between 0.5% and 50% of ivermectin. The microspheres may be formed from polylactic acid, polyglycolic acid or a copolymer of polylactic-polyglycolic acid. The programmable multi-pulse release system may also be obtained with a biodegradable matrix selected from hardenable natural polymers, such as gelatin or albumin, apart from the copolymers of lactic and gycolic acids.