Testosterone influences cell proliferation in a number of tissues (Tuohimaa & Niemi, 1968). We have produced evidence that testosterone affects the position of the 'critical decision phase' (Quastler & Sherman, 1959), or 'cut-off position' (Cairnie, Lamerton & Steel, 1965) of the small intestine of the castrated male mouse. This is the point in the crypt where the decision is made to differentiate. Any change in the cut-off position will lead to an alteration in the growth fraction (GF) or proportion of the cell population progressing round the cell cycle (Cleaver, 1967). Male Balb/c mice aged 3 months were castrated. Fourteen days later animals were given 250 μg testosterone propionate s.c. in sesame oil for 3 consecutive days. Castrated controls were given sesame oil only. Tritiated thymidine ([3H]Tdr) was administered at the same time as the third injection of testosterone or sesame oil. Autoradiographs were prepared from samples of upper