1. Experiments were performed to investigate the effects of human recombinant interleukin-1 beta on the production of vasoactive substances by human aortic smooth muscle cells in culture. Smooth muscle cells were cultured either on microcarrier beads for bioassay experiments, or in multiwell plates for the determination of nitrite levels. 2. Cells were grown on microcarrier beads, treated with interleukin-1 beta or vehicle (control) for 24 h, and packed in a column which was perfused with oxygenated Krebs-Ringer solution in the presence of indomethacin. The activity of the perfusates was bioassayed by measuring the changes in tension of a contracted ring of Wistar rat aorta without endothelium, and by evaluating the modulation of thrombin-induced platelet aggregation. 3. Perfusates from interleukin-1 beta treated cells evoked relaxations of the contracted detector tissues, and microcarrier beads covered with treated cells inhibited thrombin-induced platelet aggregation. Superoxide dismutase enhanced these effects whereas Methylene Blue abolished them. Control cells evoke neither relaxation nor inhibition of platelet aggregation. Interleukin-1 beta induced a time- and concentration-dependent production of nitrite. Cycloheximide and nitro-L-arginine inhibited the relaxations and the production of nitrite evoked by interleukin-1 beta-treated cells. L-Arginine but not D-arginine overcame the blockade elicited by nitro-L-arginine. Transforming growth factor-beta 1 reduced the interleukin-1 beta-dependent generation of nitrite by cultured smooth muscle cells and relaxation of contracted bioassay tissues. 4. Interleukin-1 beta, transforming growth factor-beta 1, Methylene Blue and L-arginine-related compounds did not induce significant variations of tension of the detector rings. 5. These data demonstrate that the inflammatory and immunological mediator interleukin-1 can stimulate the production of a nitric oxide-like substance(s) in cultured human smooth muscle cells leading to the activation of soluble guanylate cyclase. Liberation of transforming growth factor-beta by activated platelets may inhibit these reactions.