1. The possible role of protein kinase C (PKC) in homologous and heterologous desensitization of histamine H1- and ATP-receptors has been studied in monolayers of cultured vas deferens smooth muscle cells (DDT1MF-2). Cells were loaded with the calcium-sensitive fluorescent dye fura-2 and increases in intracellular free Ca2+ concentration ([Ca2+]i) monitored in response to histamine H1- or ATP-receptor activation. 2. Histamine and ATP stimulated the release of Ca2+ from intracellular Ca2+ stores and Ca2+ influx across the plasma membrane. Activation of PKC with the phorbol ester beta-phorbol-12,13 dibutyrate (PDBu; 1 microM) attenuated histamine (100 microM) and ATP (10 microM)-induced release of intracellular Ca2+ and Ca2+ influx. 3. The selective PKC inhibitor, Ro 31-8220 (10 microM), reversed the PDBu-induced attenuation of histamine (100 microM)-stimulated Ca2+ responses. 4. Histamine H1- and ATP-receptors are readily susceptible to homologous desensitization since short-term exposure to histamine or ATP (450 s) attenuated the Ca2+ responses elicited by a second application of the same agonist. Furthermore, H1-receptor activation-induced heterologous desensitization of ATP stimulated Ca2+ responses and vice versa. 5. Homologous and heterologous desensitization of histamine and ATP Ca2+ responses still occurred in the presence of the PKC inhibitor, Ro 31-8220 (10 microM). 6. These data suggest that PKC activation can attenuate histamine H1- and ATP-receptor mediated Ca2+ responses. However, based on our experimental data, PKC-independent mechanisms appear to be involved in the homologous and heterologous desensitization of histamine H1- and ATP-receptor mediated Ca2+ responses in DDT1MF-2 cells.