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Environmental consequences of treating horses with recommended dosages of ivermectin paste were studied in two controlled experiments with 29 horses in Ohio. In 1988, dung dispersal rates were measured by changes in dry weight over time of 48 copromes (300 g) formed from feces taken from four treatment and four control horses 3 days post ivermectin treatment. There was delayed dispersal of copromes from horses treated with ivermectin in June, resulting in significantly heavier ivermectin copromes compared with those of control horses by September. There was no difference in ivermectin or control copromes after treatment in August. In 1989, the effects of treating horses with oral ivermectin or oxibendazole compared with untreated controls were quantified in a study with 21 horses under natural grazing conditions after treatment on 7 June. Sequential measurements of dung pat circumferences from 20 June 1989 to 7 March 1990 showed highly significant differences between ivermectin and control pats and between ivermectin and oxibendazole pats. Reductions in mean dung pat diameters did not occur in ivermectin pats until 11 October, when there was a 4.1% reduction compared with 35.1% for control pats and 37.2% for oxibendazole pats. By the end of the study, there was a 24.7% reduction in ivermectin pats compared with 59.1% and 59.9% for control and oxibendazole pats respectively. In addition, there were significantly more pats showing complete dispersal in oxibendazole or control plots than in ivermectin plots. The calculated grazing area lost to feces was three times greater for ivermectin plots than for oxibendazole or control plots. It is suggested that environmental effects of the avermectins can be reduced by more rational use of anthelmintics.