Atypical acinar cell foci were induced in the pancreases of rats by injection of azaserine. An incubation period of 6 weeks was sufficient for the detection of all glutathione S-transferase mu positive foci. In chow-fed rats, the labelling index of foci was 12-fold higher than normal pancreatic tissue. Feeding rats raw soya flour (RSF) for up to 20 weeks did not increase the number of foci per pancreas but did produce significant increases in labelling index and growth rate. In normal pancreatic tissue, the trophic response was complete after 4 weeks of RSF feeding. In foci, however, the trophic response to RSF was prolonged. Involution of normal pancreatic tissue was seen in rats fed RSF for 19 weeks and then switched to chow 1 week prior to death. No evidence for involution was seen in the foci of these animals, although a 40-fold reduction was seen in labelling index. The labelling index of these foci was reduced to the level seen in normal tissue of chow-fed rats. These results are consistent with increased cholecystokinin (CCK) responsiveness and CCK dependence in azaserine-induced pancreatic foci.