Administration of 6 mg/kg uranyl nitrate to adult male rats resulted in a significant enhancement of N-methylnicotinamide (NMN) uptake by renal cortical slices when measured 24 or 48 h after injection. The accumulation of another organic base, tetraethylammonium (TEA), by renal cortical slices was also significantly increased by uranyl nitrate treatment, but transport of the organic acid p-aminohippurate (PAH) was not altered in these experiments. The kidney weight to body weight ratio was increased in treated rats. Accumulation of NMN by renal cortical slices was significantly enhanced within 8 h after administration of 6 mg/kg uranyl nitrate. NMN uptake was also significantly enhanced 24 and 48 h after administration of 0.5 or 1.0 mg/kg uranyl nitrate to rats. The nephrotoxicity produced by uranyl nitrate was not directly related to its induction of organic base transport. The data support the hypothesis that the organic base transport system can be selectively induced by appropriate stimuli. Treatment of rats with potassium chloride did not enhance NMN uptake by renal cortical slices.