1. The rapidly activating delayed rectifier potassium current, IKr, was studied in guinea-pig ventricular myocytes in the presence of thiopentone, which blocks the more slowly activating component of the delayed rectifier potassium current, IKs, and using whole cell perforated patch clamp or switched voltage clamp with sharp electrodes to minimise intracellular dialysis. 2. Activation of protein kinase A (PKA) by isoprenaline or forskolin caused an increase in IKr tail currents. Following a 300 ms depolarising step to +20 mV, mean tail current amplitude was increased 47 +/- 12% by isoprenaline, and 73 +/- 13% by forskolin. No increase in IKr was observed when IKr was studied using whole cell ruptured patch clamp and there was no change in the reversal potential of IKr in the presence of isoprenaline. 3. The rectification of the current sensitive to E4031, a selective IKr blocker, was markedly reduced in the presence of isoprenaline and the region of negative slope was absent. This is consistent with a reduction in the inactivation of IKr and was supported by the finding that IKr, in the presence of isoprenaline, was somewhat less sensitive to block. E4031 (5 microM) blocked only 81 +/- 5% of IKr in the presence of isoprenaline compared to 100 +/- 0% in control. 4. The forskolin- and isoprenaline-induced increases in IKr were inhibited by staurosporine and by the selective protein kinase C (PKC) inhibitor bisindolymaleimide I. Direct activation of PKC by phorbol dibutyrate increased IKr tail currents by 24 +/- 5%. Both the isoprenaline- and forskolin-induced increases in IKr were inhibited when calcium entry was reduced by block of ICa with nifedipine or when myocytes were pre-incubated in BAPTA-AM. 5. The selective PKA inhibitor KT5720 prevented the isoprenaline-induced increase in IKr only when the increase in ICa was also suppressed. 6. These data show a novel mechanism of regulation of IKr by PKC and this kinase was activated by beta-adrenoceptor stimulation. IKr seems to be enhanced through a reduction in the C-type inactivation which underlies the rectification of the channel and such a mechanism may occur in other channels with this type of inactivation.