Previous results demonstrated that incubation of the Friend murine erythroleukemic cell with 5 microM AZT for several days leads to a decrease in the rate of cell growth, inhibition of mtDNA replication, reduction of mtDNA per cell and per mitochondrion, and an increase in mitochondria per cell. As shown here, such treatment also leads to changes in lactate and ATP synthesis and in O2 uptake, suggesting impairment of oxidative phosphorylation. Direct measurement of ATP synthesis in mitochondria isolated from AZT-treated cells confirmed this view. The most significant new finding in this paper, however, is that in addition to these delayed effects of AZT, similar but very rapidly appearing effects on oxidative phosphorylation were noted, with changes observed in the above parameters including mitochondrial proliferation. Some of these occurred as early as 3 hr, only 7% of the doubling time, after exposure of the cells to 5 microM AZT, a period too short for initiation of appreciable mtDNA-mediated effects. Studies on isolated mitochondria provided no evidence of the identity of the immediate target of AZT: AZT does not act as an uncoupler or inhibitor of respiratory control, and previous results failed to implicate adenylate kinase. We have also begun to address the question of the mechanism of AZT-induced mitochondrial proliferation. Initial experiments showed that AZT inhibited synthesis of total cytosolic protein but stimulated synthesis of those proteins imported into mitochondria from the cytoplasm. We also report that aminothymidine, a catabolite of AZT in liver capable of inhibiting cell growth, was not generated by Friend cells.