The mechanism of protein kinase C (PKC) regulation by alpha-tocopherol has been investigated in smooth-muscle cells. Treatment of rat aortic A7r5 smooth-muscle cells with alpha-tocopherol resulted in a time- and dose-dependent inhibition of PKC. The inhibition was not related to a direct interaction of alpha-tocopherol with the enzyme nor with a diminution of its expression. Western analysis demonstrated the presence of PKC alpha, beta, delta, epsilon, zeta and mu isoforms in these cells. Autophosphorylation and kinase activities of the different isoforms have shown that only PKC alpha: was inhibited by alpha-tocopherol. The inhibitory effects were not mimicked by beta-tocopherol, an analogue of alpha-tocopherol with similar antioxidant properties. The inhibition of PKC alpha by alpha-tocopherol has been found to be associated with its dephosphorylation. Moreover the finding of an activation of protein phosphatase type 2A in vitro by alpha-tocopherol suggests that this enzyme might be responsible for the observed dephosphorylation and subsequent deactivation of PKC alpha. It is therefore proposed that PKC alpha inhibition by alpha-tocopherol is linked to the activation of a protein phosphatase, which in turn dephosphorylates PKC alpha and inhibits its activity.