Age at castration appears to influence growth rate of male cattle. The higher pre-weaning weight gains of bulls suggest that the male hormone is an influencing growth factor. However, post-weaning weight gains are lower in bulls castrated at eight months, which tends to nullify their pre-weaning advantage. Stress caused by the operation may have been a factor in this response. Differences in weight gains associated with diethylstilbestrol treatment were obtained whenever used in these tests. This is a progress report of experimental use of diethylstilbestrol in tests with early and late castrates to improve weight gains—a practice not presently recommended by the University of California.