The effects of leukotrienes on initiation of DNA synthesis in growth-arrested arterial smooth muscle cells cultivated in a defined, serum-free medium were studied. The results showed that LTB4, LTC4 and LTD4 were all stimulatory with a distinct effect already at 0.01 pM and a maximal effect at 10 pM; in contrast, LTE4 lacked effect. 5S,12S-DHETE, an isomer of LTB4, was inactive in itself but blocked the effect of LTB4. Treatment of the cells with indomethacin or acetylsalicylic acid, two cyclooxygenase inhibitors, blocked induction of DNA synthesis by LTB4, indicating that the effect of this compound was mediated by a cyclooxygenase product. Up to 10 pM, the leukotrienes stimulated initiation of DNA synthesis with similar potency to platelet-derived growth factor (PDGF). The maximum labelling index obtained with PDGF was, however, about twice that obtained with the leukotrienes. At suboptimal concentrations of PDGF, the leukotrienes had an additive effect. The prereplicative lag phase was 16-20 h with LTB4, 12-16 h with LTC4, and 8-12 h with PDGF. In metabolic experiments no signs of synthesis of leukotrienes were detected by cells stimulated with the calcium ionophore A23187, arachidonic acid, or LTA4 for 10-30 min, neither was any apparent degradation of LTB4 observed. On the other hand, LTC4 was transformed into LTD4 and LTE4. Taken together, the results indicate that leukotrienes are able to stimulate quiescent arterial smooth muscle cells to enter the cell cycle and to replicate their DNA. In vivo, stimulation of cell growth by leukotrienes could add to other established functions of these substances in tissue repair, inflammation, and atherogenesis.