1. This study examined the effects of diethylstilboesterol (DES), administered during and after Sertoli cell proliferation, on the testes of hatched cockerel up to the age of 20 weeks. 2. DES was injected into White Leghorn male chicks (200 ng/g body weight) over 10 d periods. The groups were first injected at 6, 8 and 10 weeks after hatching because Sertoli cell proliferation ceases at no later than 9 weeks. The birds first injected at 6 weeks and at 8 weeks were examined at 10 weeks, those first injected at 10 weeks were examined at 12 weeks and others, first injected at weeks 6, 8 and 10, were examined at 20 weeks. 3. In the birds killed at up to 12 weeks, the DES did not affect the Sertoli cell number of those first injected at week 6 and killed at week 10, although it did reduce the numerical density of seminiferous tubule containing late spermatids and increased seminiferous tubule not containing primary spermatocytes of the birds injected during weeks 6, 8 and 10. 4. In those killed at 20 weeks however, the DES did not cause any alteration in testis weight, gonado-somato index, seminiferous tubule volume, seminiferous lumen volume or comb height.