The effects of the protein kinase C activator phorbol 12-myristate, 13-acetate (PMA) on muscarinic receptor downregulation and internalization in 1321N1 human astrocytoma cells were determined. Downregulation was assessed by measuring [3H] quinuclidinyl benzilate binding to intact cells. PMA alone did not induce muscarinic receptor downregulation but instead decreased markedly both the rate and final extent of downregulation induced by the agonist carbachol. The specificity of various analogs for inhibiting carbachol-induced downregulation indicated involvement of protein kinase C. Furthermore the protein kinase C inhibitor staurosporine prevented the inhibitory effect of PMA on downregulation. In contrast, staurosporine did not inhibit agonist-induced downregulation. Neither agonist-induced downregulation nor the inhibitory effect of PMA were blocked by cycloheximide, indicating that protein synthesis is not required for these effects. Muscarinic receptor internalization was assessed both by sucrose density gradient centrifugation assays of receptor subcellular distribution and by measuring binding of the hydrophilic radioligand N-[3H]methylscopolamine to intact cells at reduced temperature. PMA did not induce muscarinic receptor internalization but rather inhibited internalization induced by the agonist carbachol. Together these results suggest that activation of protein kinase C leads to inhibition of an agonist-induced increase in the rates of muscarinic receptor internalization and degradation that are presumably responsible for receptor redistribution and eventual downregulation.