Data from 129 young Simmentaler bulls taken at the end of the Phase C growth tests, were used to study the relationship of mass and growth parameters on testes size. Scrotal circumference was positively correlated with body mass and average daily gain. Pre-weaning gain appears to positively influence testicular growth more than post-weaning gain during a feedlot test. Therefore the probability of finding bulls with relatively small testes among bulls selected for weaning mass would be relatively small. Minimum scrotal circumference at a specific age should be established for individual breeds.