Increased body:brain weight ratio in developing rats after low exposure to organic lead

Tetramethyl lead in an oily vehicle was administered to rats at weekly intervals during gestation and early postnatal life, raising the total lead concentration in the brain to about 1 microgram/g. Birth weight was unaffected, but postnatal body growth was stimulated more than brain growth, resulting in a higher body:brain weight ratio. Histological measures of brain myelination, dendritic growth, granule cell production, and retinal receptor development showed no deficit. We conclude that the body:brain weight ratio is the most sensitive of the parameters measured for detecting the effect on development of exposure to a low concentration of tetramethyl lead. The latter is neurotoxic at higher concentrations, and the stimulating effect on body growth of a low concentration is an example of "hormesis," a phenomenon which has been noted with other toxins.