Sixteen rabbits were injected with thymidine-3H and tissue sections or spermatozoa smears were evaluated by autoradiography. Two successive ejaculates were collected every second day. Analyses of labelled germ cells in twenty-four testes taken 1 hr to 32 days postinjection showed that one cycle of the seminiferous epithelium required 10·7 ± 0·5 days and that labelled spermatozoa should enter the caput epididymidis 31·4 days post-injection. Labelled spermatozoa were found in the proximal portion of four capita epididymidum 32 days and one caput 33 days post-injection, but not in eleven other capita epididymidum 28 to 31 days post-injection. Labelled spermatozoa first were detected in ejaculated semen from five rabbits on Days 38, 39, 41, 42 and 42. The mean number of grains/spermatozoon in autoradiograms of successive samples agreed closely with the scintillation count activity curve. However, neither curve represented the excretion pattern of spermatozoa labelled as preleptotene spermatocytes. The first samples containing labelled spermatozoa had a low percentage of highly labelled spermatozoa while, through mixing with spermatozoa labelled as spermatogonia, subsequent ejaculates contained up to 55% labelled spermatozoa but individual cell activity was lower. Labelled spermatozoa were excreted over a period of 2 to 3 weeks. Epididymal spermatozoa transit time averaged 9·0 days, but it ranged from 6·6 to 10·6 days even with a uniform semen collection schedule. Combined with autoradio--graphic evaluation of tissue sections or semen smears, intravenous injection of 0·1 to 0·3 mc/kg body weight of thymidine-3H should be adequate for experiments of this type.