Most studies on lead toxicity in the suckling rat have been performed with doses leading to growth retardation. In a previous paper ( Sundstr öm et al. 1983), the effects of different lead doses on normal suckling rats were described. The dose of 10 mg/kg body weight daily given on days 1-15 pp produced minute hemorrhagic lesions on day 15 in the cerebellum, whereas rats given 5 mg/kg body weight daily lacked microscopically discernible pathologic changes in the brain. None of these groups exhibited growth retardation. To further elucidate the association between lead encephalopathy and malnutrition, lead was administered to protein-deprived suckling rats. Protein deprivation was achieved by a diet with 50% reduction of protein content. The mothers of the pups were fed this diet from 2 weeks before conception throughout the experiment. Experimental animals were injected i.p. with 5 mg or 10 mg lead nitrate/kg b.wt. daily. Littermates, injected with vehicle without lead nitrate served as controls. Protein-deprived rats without either treatment were "external" controls. Animals were killed at 10, 15, and 20 days age for determination of lead content in blood and brain and for light-microscopic examination. The protein-deprived rats given 10 mg/kg b.wt. daily were growth-retarded as compared to unexposed protein-deprived rats. The mortality was almost 100% at 15-20 days pp. At 15 days, the cerebellum of these rats showed abundant hemorrhages, and the cerebrum was also hemorrhagically discolored. Protein-deprived rats given 5 mg/kg b.wt. daily did not differ significantly from unexposed protein-deprived rats with regard to body weight gain.(ABSTRACT TRUNCATED AT 250 WORDS)