Leukotriene synthesis and metabolism were studied in cultured porcine aortic smooth muscle cells (PSM). Cultures stimulated with calcium ionophore A23187, with or without exogenous arachidonic acid, did not release detectable levels of leukotriene B4, C4, D4 or E4. Those products were assayed by high-performance liquid chromatography, ultraviolet spectrometry and, in some cases, radioimmunoassay. Smooth muscle cultures were able to convert leukotriene A4 to leukotriene C4, indicating the presence of leukotriene C4 synthetase. Although this enzymatic activity has previously been found in cultured porcine aortic endothelial cells, it was not detectable in cardiac myocytes, fibroblasts from several organs or renal epithelial cells. It is known from previous work that inflammatory cells such as polymorphonuclear leukocytes (PMNL) or mast cells release leukotriene A4 when stimulated. Further, increased numbers of these cell-types are found associated with vascular tissue during several pathologic situations. Therefore, the potential for a leukocyte-smooth muscle cell interaction involving the transcellular metabolism of leukotriene A4 was assessed. Stimulation of PMNL suspensions in the presence of PSM resulted in a significant increase in total leukotriene C4 produced in comparison to either cell-type alone (255% of PMNL alone, P less than 0.05). Furthermore, after the intracellular glutathione pool of PSM was prelabelled with 35S, a PSM-PMNL coincubation produced levels of [35S]leukotriene C4 which were significantly greater (P less than 0.05) than those found after coincubating prelabelled PMNL with unlabelled PSM. These data demonstrate a PMNL-PSM interaction in which smooth muscle cell leukotriene C4 synthesis results from the transcellular metabolism of PMNL-derived leukotriene A4. Since leukotriene C4 and its metabolites are vasoconstrictors and cause increased vascular permeability, the biochemical interaction described in this report may be relevant to the pathophysiology of arterial vasospasm, atherogenesis and to the abnormalities of tissue perfusion associated with ischemic or inflammatory disorders.