The antiviral effect of azidothymidine (AZT) can be potentiated by acyclovir (ACY), and this drug association has been used m the management of HIV-infected patients. In the present study we examined the effects of this association on the livers and kidneys of both pregnant rats and their concepts. Previous data from this laboratory suggested that the deleterious effects of ACY on rat pregnancy are due to its extraplacental actions and these are, at least in part, counteracted by concomitant treatment with AZT Kidneys and livers of pregnant rats were noticed to be much more sensitive to the toxic action of the drugs than those of their concepts, ACY eliciting much more evident morphological alterations than did AZT Contrary to what was expected, in the group of rats treated with both drugs AZT was not able to diminish the severity of the alterations evoked by ACY. The proposed "protective" action of AZT against the abortive effect of ACY on rat pregnancy does not seem to be exerted through a renal or hepatic pathway.