Many estimates have been made of sperm production in various species, but in most cases examinations have been made infrequently or with irregular intervals between successive collections. Since during a period of sexual rest spermatozoa may accumulate in the epididymis, or with longer periods may undergo disintegration and absorption in the vas and ampulla (Simeone & Young, 1931), the number ejaculated at an particular collection will not necessarily reflect the level of sperm production in the testis. Moreover, there is the possibility that copulation may actually stimulate spermatogenesis. In horses, Chang (1943) found that more sperms were ejaculated if collections were made frequently and at regular intervals. In the present experiments an attempt has been made to determine the effect of frequency of collection upon the sperm production of rams, the collections being taken at regular intervals and for a definite period, in this case ten days.