Ag/AgCl and Ir-coated electrodes allow the recording of the monophasic action potential (MAP) due to their electrical properties like non-polarisability. This study investigates the correlation of MAP recorded with both types of electrodes. In 20 mongrel dogs (18 +/- 6 kg) an Ag/AgCl and an Ir-coated catheter (Ir) were placed endocardially in the apex of the right ventricle. The effects of isoproterenol and verapamil were investigated during spontaneous rhythm and stimulation simultaneously recorded with both types of electrodes in 10 dogs without AV-node ablation. The correlation at different heart rates were investigated in 10 other dogs with complete AV-block. The morphology and amplitudes of MAP were comparable (AgCl: 15 +/- 7 mV; Ir: 13 +/- 8 mV). Following an i.v. bolus of 2 micrograms/kg isoproterenol the spontaneous rate increased (175 +/- 18 to 245 +/- 25 bpm). During stimulation with 250 ms cycle length the duration shortened (MAPd90: AgCl: 160 +/- 11 to 130 +/- 12 ms; Ir: 154 +/- 18 to 128 +/- 15 ms). The alterations reversed after 20 min. An i.v. bolus of 0.2 mg/kg verapamil decreased the spontaneous rate (167 +/- 11 to 104 +/- 23 bpm) and lengthened the MAPd90 (AgCl: 182 +/- 14 to 220 +/- 13 ms; Ir: 174 +/- 16 to 216 +/- 21 ms) at 300 ms stimulation. The correlation between the MAPd90 of both lead types was r = 0.98 during all measurements. Under the effect of beta-agonist and Ca(2+)-antagonist medication MAP showed a strong correlation recorded with both types of electrodes. Thus, both leads allow the recording of MAP but only the Ir-electrodes with their long-term stability are implantable and allows us to control the effects of drugs with implantable devices.