Incubation of IIC9 fibroblasts with lysophosphatidic acid (LPA) induced an increase in the amount of filamentous actin (F-actin), which was concentration-dependent with a maximal effect at 100 ng/ml. Phosphatidic acid (PA) also produced a concentration-dependent increase of F-actin, but it was less potent than LPA. The LPA-induced increase in F-actin was rapid and sustained for at least 60 min. LPA rapidly increased the levels of PA and choline, with maximal increases at 5 min and 30 s respectively. LPA also caused a monophasic increase in diacylglycerol (DAG) which lagged behind the increases in PA and choline. LPA stimulated phosphatidylbutanol formation in the presence of butanol and produced a small increase in inositol phosphates that was much less than that induced by alpha-thrombin. Pretreatment of cells with pertussis toxin (PTX) caused greater than 50% inhibition of the LPA-stimulated increases in PA, DAG and choline. PTX increased the LPA concentration required to induce half-maximal actin polymerization by about 10-fold. PTX caused a similar shift in the dose-response curve for LPA-induced PA formation. These results suggest that LPA induces an increase in PA by activating a phosphatidylcholine-hydrolysing phospholipase D via a PTX-sensitive G-protein and that the increase in PA is involved in the activation of actin polymerization.