Restricted medication experiments were done to correlate time of arprinocid medication with developmental stages of the life cycle of Eimeria tenella. By the criterion of histopathology and using a massive inoculum (10(6) sporulated oocysts), 50 ppm was partially active and 70 ppm was fully active against the first asexual generation when medication was delayed until day 1. When medication was delayed until day 2, full activity was demonstrated against the late, first asexual generation. When medication was delayed until day 3, definite but less complete activity was shown against the late, second asexual generation. Using the conventional efficacy parameters and with an inoculum of 5 X 10(4), full activity occurred with 50 ppm when medication was started up to day 2. Essentially full activity was observed with 60 ppm started at day 3. The combined results of the two tests are interpreted to indicate high activity against both the first and second asexual generations. Medication with 70 ppm changed the wall-forming bodies of the macrogamete. They were indistinct and less intensely eosinophilic than controls.