Few studies have investigated the potential developmental differences resulting from treating neonatal rat pups in either split-litter or whole-litter (nested) experimental designs. We directly compared rat pups dosed with triethyl lead (TEL) via both split-litter (representing all dosage groups within a single litter) and nested (all pups randomly assigned to a single litter receive the same dose) designs. The nested design was chosen to produce a uniform behavioral pattern across pups within each litter, whereas the split-litter design was chosen to promote pup competition and differential maternal care. On postpartum day 5, pups were administered either 12, 13, 14, or 15 mg/kg TEL, with each design represented by 12 litters. Although the LD50 values for the two designs were not significantly different, there were significantly more deaths in the 12 mg/kg dosage group within the split-litter design than in the nested design group. Preweaning survival times for split-litter dosed animals were also decreased. In addition, significant growth reduction (7-16%) was observed in the split-litter group, relative to the nested design animals during the preweaning period. These results suggest that neonatal toxicity is not independent from experimental design considerations, and that the factors of littermate competition and/or pup-induced maternal care deserve further study.