To characterize the effects of combined treatments with gamma radiation and diethylnitrosamine (DEN) on the induction of histochemically detectable altered hepatocyte foci and hepatic tumors, we assessed the yields of these lesions in the livers of 150-day-old rats that had been treated neonatally with a single dose of gamma radiation (75 rad, whole body) and i.p.-injected DEN (0.15 mumol/g body wt), either separately or in combination. The combined treatments involved the administration of the two stimuli in both possible sequences, with the interval between treatments set at 1 h. The focus population was examined for two histochemical markers (elevated gamma-glutamyl transpeptidase [GGT(+)] and iron exclusion [FE(-)], giving rise to three detectable focus phenotypes, i.e. GGT(+) foci, FE(-) foci, and GGT(+), FE(-) foci. Frequencies of the three phenotypes were quantitated through the use of serial frozen sectioning techniques and computer-assisted image analysis. GGT(+) focus induction was synergistically enhanced by the combined treatment irrespective of the order in which the two stimuli were administered; the remaining two phenotypes did not show such enhancement. The magnitude of the GGT(+) focus response was significantly greater when the treatment sequence was gamma----DEN as opposed to DEN----gamma. Tumor yields in rats receiving combined gamma--DEN treatment were similar to those in rats receiving the DEN alone, irrespective of the gamma--DEN treatment sequence. These results suggest that phenotypically distinguishable lesions, including foci with different histochemical marker patterns and tumors, originate from specific types of damage at different genetic loci and are developmentally independent; and the expression of the GGT(+) marker per se in altered hepatocyte foci is not a reliable index of incipient hepatic neoplasia.