AbstractThe activities of an endogenous inhibitor and of a stimulator of cell proliferation were assayed in the livers of sham‐operated (SO) or partially hepatectomized (PH) adult rats; rats fed a choline‐supplemented (CS) or a choline‐devoid (CD) diet; the same diets followed by acute CCU intoxication; the same diets supplemented with phenobarbital (PHB) or a CD diet containing DL‐ethionine (ETH). The inhibitor and the stimulator were semipurified by fractional ethanol precipitation of a liver cytosolic fraction, and their activities were assessed by means of bioassays in vitro. The livers of SO rats and of rats fed the CS diet contained only inhibitor activity. Following PH, a CD diet, or CCU intoxication the inhibitor activity was suppressed, and there was a simultaneous appearance of a stimulator activity. Thus, PH, a CD diet, and CCU intoxication cause similar cellular (loss and regeneration) and humoral‐homeostatic changes in adult rat livers. We propose that these changes constitute a basic attribute of the mechanism whereby the three conditions affect similarly hepatocarcinogenesis in the rat, especially in the case of a CD diet, because the changes it induces are chronic rather than acute. PHB, another promoter of chemical hepatocarcinogenesis, affected neither the inhibitor nor the stimulator activity. Thus, PHB seems to be acting by a different mechanism than that of the other three agents. ETH did not modify the shift in the balance of the growth‐modulating factors induced by a plain CD diet. This shift may account for the marked stimulation of carcinogen‐induced oval cell proliferation exerted by a CD diet. The significance of these results is discussed in the context of known effects of a CD diet and of PHB on hepatocarcinogenesis in rats.