The separate and combined effects of protein deprivation and nitrofen exposure were studied in the pregnant rat. Animals were fed diets containing 24, 8, 6 or 4% casein throughout gestation. Within each diet group, subgroups were gavage-fed with 12.5 (lower dose) and 25 (higher dose) mg nitrofen/kg body weight or with oil carrier only on days 7-21 of gestation. Dams were weighed and food intake was measured daily. On day 21 of gestation, cesarean-derived pups were examined for congenital anomalies and dissected for determination of organ weights. Skeletons were alizarin-stained and examined for skeletal anomalies and developmental stage. No effects on the skeleton or gross congenital anomalies were seen. Fetal size and weights of liver, kidney, intestine, heart, lung and brain were reduced with decreasing casein content of the diet and as a result of the higher dose of nitrofen. An effect of interaction between diet and nitrofen exposure was shown in the kidney, intestine and lung weights. Specific toxicity affecting organ size was shown to occur in the intestine and lung. An interaction between diet and nitrofen specifically affected kidney and intestine. Brain size tended to be preserved, a possible protective mechanism negated by nitrofen exposure. The data suggest also that kidney, intestine and lung are particularly affected. Effects, however, occur primarily in young of severely malnourished dams receiving a relatively high dose of nitrofen.