Protein kinases are known to undergo phosphorylation to regulate their activity. To determine whether the protein kinase activity of p37v-mos was similarly regulated, we investigated the influence of two well known protein kinases, namely protein kinase C and protein kinase A, on the activity of p37v-mos in vivo. NIH3T3 cells chronically transformed with Moloney murine sarcoma virus 124 were treated with high concentrations (200-400 nM) of phorbol 12-myristate 13-acetate (PMA) for 24-48 h, concentrations known to result in the total loss of protein kinase C by causing its translocation from the cytosol to cell membranes where it is downregulated. PMA treatment caused a drastic decrease in the protein kinase activity of p37v-mos without affecting its steady state level. Similar results were obtained with p85gag-mos expressed in ts110 Mo-MuSV transformed NRK cells. Control treatment with an inactive analogue of PMA, 4-alpha phorbol 12,13-didecanoate, had no effect on the p37v-mos protein kinase activity. Treatment of cells with a direct chemical inhibitor of protein kinase C, H-7 (1-(5-isoquinoline sulfonyl)-2-methylpiperazine dihydrochloride), approximately halved p37v-mos kinase activity, although the drug did not inhibit p37v-mos kinase activity directly in vitro. In contrast to the PMA effect, in vivo activation of protein kinase A by 8-(4-chlorophenylthio)-adenosine 3',5' cyclic monophosphate did not affect p37v-mos protein kinase activity levels. These findings indicate that the protein kinase C pathway but not the protein kinase A pathway modulates v-mos protein kinase activity.