The aim of the present investigation was to evaluate the in vitro (10-500 microM) and in vivo (1-21 subcutaneous injections of 2.5 mg/kg each) effects of HgCl2 on the ATP diphosphohydrolase activity (EC 3.6.1.5; apyrase) of synaptosomes from cerebral cortex of rats at different ages (5, 11, 18 and 25 days of life). The in vitro results showed that HgCl2 (from 10 to 500 microM) inhibited the hydrolysis of both substrates by the synaptosomal enzyme at all ages studied. In contrast, HgCl2 injected in vivo did no affected the normal ontogeny of ATP and ADP hydrolysis. The hydrolysis of both nucleotides increased at the same rate as a function of age in control and HgCl2-treated rats (the specific activity of enzyme increased about 5-fold from the first week of postnatal life of weaning). The results of the present study demonstrated that in vitro HgCl2 inhibited the enzyme, but was ineffective when tested in vivo. Probably the absence of an in vivo effect is due to the low permeability of blood-brain barrier to inorganic forms of mercury.