The cell cycle properties of undifferentiated spermatogonia in the Chinese hamster were analysed by the fraction of labelled mitoses technique (FLM) in whole mounted seminiferous tubules. The minimum cell cycle time (Tc) was found to be c. 90 hr for the As and 87 hr for the Apr and Aal spermatogonia, which is appreciably longer than for the differentiating types A2-B2 spermatogonia (60 hr). This is mainly accounted for by a longer tG1. In general the variability in the duration of the cell cycle phases is greater than for differentiating spermatogonia. From the shape and position of the second peak of the FLM curve it could be concluded that the undifferentiated spermatogonia either cycle with a Tc of c. 87-90 hr, or become arrested in G1. This implies that the decrease in proliferative activity of the undifferentiated spermatogonia after stage IV takes place by the arrest of progressively more cells, i.e. by a gradual decrease of the growth fraction, and not by a gradual lengthening of tG1. The arrested cells either differentiate into A1 spermatogonia and divide in stage IX, or remain undifferentiated and are stimulated to enter S again during the following epithelial cycle. It could be deduced from the heights of the second FLM peaks of As and Apr spermatogonia that once triggered into active cycle, the daughter cells of As spermatogonia that became Apr have a greater chance to continue cycling than those that became new As cells.