Groups of 15 male and 15 female rats were given diets containing 0 (control), 0·2, 1·0 or 5·O% diethyl phthalate (DEP) for 16 wk. Consumption of diet containing 5% DEP was associated with a reduction in food intake and in the rate of body-weight gain, and similar effects were seen in the females given 1% DEP. No statistically significant effects on water intake or on the results of the haematological examinations, serum-enzyme levels, urinary cell-excretion rate, renal concentration tests or histological examination were seen in the treated animals. However, there were increases at wk 16 in the relative liver weight of females at all treatment levels and of males fed the highest level. There were also increases in the absolute and relative weights of stomach and small intestine in rats of both sexes; in the females these were statistically significant at all dietary levels at wk 16.