The time parameters of the cell cycle were determined in the jejunal crypts of male Balb/c mice by an FLM experiment. The cell cycle time was 12.42 +/- 0.11 h, and the duration of DNA synthesis was 7.61 +/- 0.99 (mean values +/- S.E.). A stathmokinetic technique using vincristine gave a value of Tc of 11.8 h. The growth fraction (Ip) calculated from cycle parameters and the labelling index was 0.61, while a value of Ip estimated from a labelling index distribution curve was 0.65. For the whole crypt a value of 0.86 h was obtained for the duration of mitosis, longer than that in the rat. The mitotic duration was found to vary with cell position, but values of between 0.8 and 1.2 h prevailed throughout the proliferative compartment; values in the basal cell positions appeared shorter. Apparent cell cycle times were longest in the basal cell positions. A value for crypt migration rate calculated from a cumulative birth rate curve was 1.48 cell positions per h, compared with a value of 1.8 +/- 0.27 cell positions per h as measured from movement of the 50% peak value on the labelling index distribution curve with time after tritiated thymidine. The crypt cell production rate was calculated from microdissected and squashed crypts to be about 14 cells per crypt per h. There was a total crypt population of 282 +/- 65 (S.E.) cells, of which 172 were proliferative.