The effects of Chinese medicine, alcoholized rhubarb, on cardiac electrophysiology and contractile force were studied by using monophasic action potential (MAP) techniques with an improved pressure-electrical transducer epicardial electrode and/or an endocardial pressure-contact electrode catheter in 27 feline hearts in vivo. The results of the experiments were as follows: 1) Alcoholized rhubarb solution (1.5 g/kg) caused a greater increase in the MAP amplitude (from 18.8 +/- 0.62 mV to 26.7 +/- 3.87 mV, p < 0.01), in the MAP maximum rate of rise of phase 0 (from 1.7 +/- 0.13 V/sec to 2.6 +/- 0.43, p < 0.01), and in the MAP duration at 50% and 90% repolarization (from 194.3 +/- 14.00 msec to 216.3 +/- 6.30 msec and from 238.8 +/- 21.41 msec to 280.5 +/- 14.68 msec, respectively, p < 0.01), and a decrease in the heart rate (from 130.8 +/- 12.10 bpm to 110.8 +/- 10.35 bpm, p < 0.01) in the epicardial monophasic action potentials after rhubarb solution administration for 1 min. 2) The ventricular contractile force of the feline heart in vivo was also significantly increased (11.1 +/- 0.52 g to 14.0 +/- 1.41 g, p < 0.01) by rhubarb solution. 3) A dose-dependent relation of alcoholized rhubarb solution was found and indicated that lower doses (1.0-1.5 g/kg) had an excitatory effect, while a higher dose (2.0 g/kg) had inhibitory or toxic effects on the intact feline heart. The present study provides evidence that Chinese rhubarb solution has electrophysiological effects on the feline heart in vivo.