Testes were collected from normal and cryptorchid horses of a variety of breeds and ages and weighed after dissection from the epididymis. Scrotal testes grow little until the second winter of life and little thereafter, although a nearly mature body weight is reached by the end of the first winter. Scrotal testes in unilateral cryptorchids tend to be larger than those of normal stallions, sometimes exceedingly so, although occasional small scrotal testes are recorded. Inguinal testes show some tendency to grow during the second winter but the data are difficult to analyse beyond that age because of the transient nature of most cases of inguinal cryptorchidism. Abdominal testes do not appear to increase in size with age but if a unilateral abdominal cryptorchid has its scrotal testis removed, the remaining abdominally retained testis undergoes marked hypertrophy.