We have analyzed the effect of transfection into murine NIH/3T3 cells of the human dsRNA-activated kinase PKR on the expression of the β-galactosidase reporter gene, placed under control of the HIV1 or the HTLV-I LTR. β-Galactosidase expression is stimulated when the reporter plasmids are cotransfected with wild-type PKR but inhibited when cotransfected with a catalytically inactive mutant PKR. In the case of HIV1, β-galactosidase expression was not stimulated when cotransfection was carried out with PKR harboring mutations in the dsRNA binding domains, indicating that stimulation depends on the classical mode of PKR activation through dsRNA binding. In contrast, the dsRNA binding mutants of PKR could still partially stimulate β-galactosidase expression from the HTLV-I LTR, suggesting that PKR activation in this case may involve different/additional mechanisms. These results show that, in addition to the known down-regulation of protein synthesis through eIF2 phosphorylation, PKR can also positively stimulate gene expressionin vivo, most probably through phosphorylation of a substrate distinct from eIF2.