Male rats were starved for a period of 96 hours. Measurements of crypt cell population showed a small reduction during starvation. The growth fraction remained constant, but the total number of proliferating cells per crypt fell as a consequence of the reduction in crypt population. Both labelling and mitotic indices fell throughout the starvation period. The cell cycle time (Tc), measured by a stathmokinetic technique using vincristine, was increased from 10-4 hours in control rats to 14-7 hours after 96 hours' starvation. Upon refeeding, the proliferative indices were observed to rise. After a small initial fluctuation, the growth fraction remained constant. The crypt population remained substantially unchanged. Sixteen hours after refeeding, the cell cycle time was reduced to 6-5 hours. The hypoproliferative response to starvation is mediated solely by an increase in cell cycle time, and the response to refeeding is interpreted in terms of a reduction in Tc. Changes in the size of the proliferating population are considered not to play an important role in either response, although it is not possible to exclude entirely the presence of resting cells in the proliferative compartment itself.