Using floating microelectrode, we have been able to record the transmembrane potential of the ventricular cells of in-situ perfused heart of the tortoise and observed the effects of high potassium and high calcium on it. It was found that high potassium(8.0 mM) could lower resting potential (Rp), reduce action potential amplitude (APA), and shorten action potential duration at 50% and 90% of repolarization (APD₅₀, APD₉₀). High calcium (3.2 mM) could lower RP, increase overshoot (OS), shorten APD, but had no significant influence on APA. It could antagonize the effect of high potassium on heart rate (slowing heart rate) and APA, but act synergistically with high potassium on shortening of APD. The influence on heart rate indicates that there is an antagonistic effect on the pacemaker cell of the sinus venosus between potassium and calcium.