The anthelmintic activity of albendazole (5 mg/kg) and cambendazole (20 mg/kg) was compared in twenty-two naturally infected horses and ponies, which were divided into two equal groups. Comparison was based on the faecal strongyle egg counts. On the fourteenth and twenty-eighth days after the first treatment in May, the reductions in faecal egg counts were 99.1 and 93.9 per cent respectively in the albendazole group and 88.0 and 75.7 per cent respectively in the cambendazole group. Within fourty-two days after treatment had been initiated, the mean EPG (eggs per gram) had been restored to the initial level in both groups. The same animals were treated again in August; this time, however, the anthelmintics were changed. In the albendazole group, the faecal egg counts were reduced by more than 95 per cent and, in the cambendazole group, by not more than 84.1 per cent. As the horses were not weighed, the disappointing results obtained with camendazole may have been due to low a dosage, although resistance cannot be ruled out. When the trial was concluded in October, within fifty-six days after treatment in August, the faecal egg counts continued to be remarkably low in both groups: 8.3 per cent of the average EPG prior to treatment in the albendazole group and 21.2 per cent in the cambendazole group. As the trials did not provide any evidence to suggest a larvicidal action of the two anthelmintics, the difference with the results of treatment in May is attributable to an inhibitory effect on the development of the infective Strongyle larvae, specifically Cyathostominae, ingested during the summer.