The effects of postnatal methylmercury exposure on the ongoteny of kidney and liver responsiveness to trophic stimuli were examined. Increased ornithine decarboxylase (ODC) activity was used as an index of tissue stimulation. In the rat, kidney ODC responsiveness to growth hormone, angiotensin, vasopressin, isoproterenol and serotonin is absent at birth and matures 3 to 4 weeks later. However, pups exposed to methylmercury showed marked ODC responses to these same agents as early as 10-19 days of postnatal age, accompanied by a significant renal hypertrophy. In contrast to the kidney, the liver of normally developing rats is responsive to trophic factors even in the neonate. In this tissue, there was no consistent effect of neonatal methylmercury treatment on ODC responses at any developmental stage tested; although absolute liver weights were reduced, liver/body weight ratio was not affected. These results demonstrate that postnatal methylmercury exposure causes organ-specific alterations in the ontogeny of tissue ODC responsiveness to trophic stimuli, which may participate in subsequent effects of this organomercurial on overall tissue development and function.