Feeding a diet of raw soya flour, which contains a trypsin inhibitor, results in pancreatic hypertrophy and hyperplasia, and when this diet is withdrawn rapid involution ensues. This study examines whether cells produced during the hyperplastic response to raw soya four (RSF) are selectively destroyed during involution following the withdrawal of this diet. Six week old male Wistar rats were fed RSF. At 7, 9 and 12 d after commencing the diet, during the period of maximum cell proliferation, rats were injected with 0.5 muCi/g body weight of tritiated thymidine. The rats were continued on this diet for a further 16d. By 1 mth pancreatic DNA, RNA, protein, weight and the specific activity of pancreatic DNA were all significantly greater in rats fed RSF, compared to control rats fed a standard diet. When rats fed RSF were changed to a diet free of trypsin inhibitor for 7 d, pancreatic DNA, RNA, protein and weight returned to control values, however the specific activity of DNA remained unchanged from the RSF value. This report suggests that cell death involves cells produced before and during the hyperplastic response to RSF, since there is no change in the specific activity of the organ after involution.