The M current, IM, a voltage-dependent noninactivating K current, was recorded in NG108-15 neuroblastoma x glioma hybrid cells, using the whole-cell mode of the patch-clamp technique. We studied inhibition of the M current by bradykinin, phorbol dibutyrate (PDBu), an activator of protein kinase C (PKC), and methylxanthines. Focal application of 0.1 -5 gM bradykinin inhibited I M by about 60%; 5 nM bradykinin inhibited by about 40~ Bath application of 0.1 gM and 1 gM PDBu diminished I M to about half of the control value. Staurosporine, a PKC inhibitor, applied for 35-43 min in a concentration of 0.3 gM significantly reduced the effect of i txM PDBu. M current blockage by PDBu could be partly reversed by bath application of H-7 (51 -64 ~M), another PKC inhibitor. These observations suggest that the PDBu effect is really due to activation of PKC. The findings are compatible with the view [Brown DA, Higashida H (1988) J Physiol (Lond) 397:185-207] that the bradykinin effect on IM is mediated by PKC. However, three further observations suggest that this is only true for part of the bradykinin effect. When the suppression of I M by I~M PDBu was fully developed, 0.1gM bradykinin produced a further inhibition of I M. Downregulation of PKC by long-term treatment with PDBu reduced the effect of 0.1 gM bradykinin significantly but did not abolish it. Staurosporine (0.3 gM, applied for 31-46min) failed to reduce the effect of 5nM bradykinin significantly. The M current could be reversibly blocked by methylxanthines (caffeine, isobutylmethylxanthine, theophylline) in the millimolar range, probably because of a direct action on the M channels.