Astroglial cells are known to proliferate during development of the nervous system, as well as during post-traumatic gliosis. We have previously shown that the proliferation of cultured astrocytes can be stimulated by the urokinase-type (uPA) of plasminogen activator (PA) and that astrocytes are able to release such uPA upon stimulation with basic fibroblast growth factor, which is known to act as a mitogen for these cells. Here we report studies on the effects of human interleukin-1 (IL-1) on the release of PA activity by cultured newborn rat astroglial cells. Whereas there is controversy in the literature as to whether IL-1 stimulates multiplication of astroglial cells, we failed to observe such an effect in our system. We did observe, however, a dose-dependent decrease in PA activity in the supernatant of the IL-1 treated cultures. Further analysis revealed that this apparent decrease in PA release was in fact due to an increased release of plasminogen activator inhibitor (PAI). A similar IL-1 induced increase in PAI release was also found to occur in cultures of transformed astrocytes (human glioma LN18) and in cultured Schwann cells, but not in cultures of neurons or neuronal tumour cells. Since protease inhibitors are known to possess neuritogenic properties, our results suggest that IL-1, by its capacity to induce PAI, may promote neuritogenesis.