In the present study evidence is provided for the existence of phospholipase D (PLD) activity in rat Leydig cells. Leydig cells were cultured and labelled with [H]myristic acid. In the presence of ethanol, phorbol 12-myristate 13-acetate (PMA) stimulated the formation of [H]phosphatidylethanol ([H]PEt) in a dose-dependent manner at the expense of [H]phosphatidic acid ([H]PA). In cells prelabelled with [H]choline, PMA caused a rapid increase in intracellular free [H]choline. The time course of [H]PEt formation was similar to the time course of intracellular [H]choline formation. The data taken together support the notion that PMA stimulates phosphatidylcholine (PC) hydrolysis by a mechanism, which principally involves PLD. Activation of PLD by PMA was inhibited by long-term pretreatment of cells with PMA to downregulate protein kinase C (PKC) and by pretreatment with staurosporine. These data support the notion that activation of PLD by PMA is dependent on PKC. Arginine vasopressin (AVP) caused a rapid stimulation of PLD activity in the cells. This activation was inhibited after downregulation of PKC, indicating that the agonist acts by a mechanism similar to that of PMA.