Cell cycle parameters in the dentate gyrus of the postnatal rat hippocampus were measured, using the method of percentage labelled mitoses. In animals aged 1, 6 and 12 days, cell cycle times range from 15-1 to 17-7 h, S phase duration from 10-1 to 11-7 h, length of the G2 phase from 2–5 to 3-3 h and G1 from 1-1 to 2–4 h. These figures are similar to those obtained for other precursor cell populations in the brain. On the day after birth the labelling index in the dentate hilus is 4–5% and estimated turnover time 220 h, while on day 6 the labelling index is 3–7% and estimated turnover time 310 h. It is calculated that precursor cells outnumber granule cells by at least 3:1 at birth and 2:1 by day 6.