Pulse labelling experiments with [3H] thymidine (dT) and double labelling experiments with [3H]dT and bromodeoxyuridine (BrdUrd) were carried out on cells of the subependymal layer in the brain of adult normal mice in vivo, in vivo/in vitro and in vitro. The results should (i) lead to information about cell cycle parameters of these cells in the brain of adult mice, since these cells have been studied mostly in the rat brain up to now and (ii) answer the question whether results concerning cell proliferation obtained in vivo correspond with those from brain slices incubated in vitro with or without prelabelling in vivo. In vivo an LI of 20.2 +/- 2.7% (mean +/- SEM) and Ts = 7.2 +/- 0.7 h were found. Furthermore, grain count halving experiments led to a surprisingly short cycle time (Tc) of 11.2-14.2 h. The longer Tc values (18-20 h) reported in the literature for subependymal cells in the rat brain seem to be due to evaluations of different areas around the lateral ventricle without considering the migrating behaviour of these cells which is quite different regionally. The in vitro studies (with or without prelabelling in vivo) showed a significantly reduced LI due to the fact that about 20% of the S phase cells, possibly lying in the middle of S, stopped further DNA synthesis after transfer to culture. This was shown by comparing the cell fluxes at the G1/S and S/G2 borders of in vivo vs. in vitro studies.