Ninety rats of the Wistar strain were divided into 18 groups (5 animals in each). Stannous chloride was administered to animals by different routes (subcutaneously, intraperitoneally, intragastrically). Zinc was given intraperitoneally and lead was given subcutaneously, both every other day. Control animals received 0.9% NaCl. Aminolevulinic acid dehydratase (ALAD) activity was clearly decreased due to the double tin dose (total dose 4 mg Sn/kg), whereas 7 doses (altogether 14 mg Sn/kg) resulted in almost complete enzyme inhibition. ALAD inhibition in rat blood was similar, whatever the SnCl2 administration route. Greater inhibition was noted when this enzyme measured at pH 6.8. Zinc administered intraperitoneally (without tin) was also found to inhibit ALAD activity. The zinc doses applied did not protect ALAD activity against inhibition by tin, administered either intragastrically or intraperitoneally.