These studies have provided evidence that DEHP and DEHA do not bind covalently to DNA and do not therefore possess the characteristics of a genotoxic agent (Lutz, 1982). This suggests that the tumours induced in the rodent liver may result from some non-genotoxic mechanism and supports the view that the weakly positive dominant lethal test seen on administration of DEHP by the ip (but not the oral) route (Singh et al. 1974) is unlikely to have resulted from a direct effect on the genome of the sperm cells. Although the mechanism responsible for the induction of tumours by high doses of DEHP in rodents is not clear, it would appear both from these studies and from work on hypolipidaemic agents, that peroxisomal proliferation and the induction of enzymes associated with this organelle are in some way implicated (Cohen & Grasso, 1981). Other studies have shown that changes of this type are produced by doses of hypolipidaemic agents that induce liver cancer in rodents (Cohen & Grasso, 1981) and our investigations have indicated that they were also prominent at dose levels of DEHP similar to those that induced liver cancer in the NCI study (National Toxicology Program, 1982). No cancer induction would be expected to occur in the absence of these changes. In our dose-response study in rats it was shown that at the lowest dose (50 mg/kg body weight/day, approximately equivalent to a dietary level of 1000 ppm) several effects seen with higher doses were not apparent and others differed only slightly from normal control values. This is particularly relevant to assessments of the risk posed by DEHP and DEHA present as contaminants in foods, since human exposure via the food chain has been estimated by Shiota, Chou & Nishimura (1980) as 30 micrograms/kg body weight/day, several orders of magnitude less than the lowest exposure level used in these experiments. In addition, our studies indicate that none of the changes found in the rat were observed in the marmoset, suggesting that rodents and primates differ fundamentally in their hepatic and testicular response to DEHP. Previous studies by other authors (reviewed by Cohen & Grasso, 1981) indicated that morphological changes in the endoplasmic reticulum and the proliferation of peroxisomes are not features of the response of monkeys and man to high doses of hypolipidaemic agents.(ABSTRACT TRUNCATED AT 400 WORDS)