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The effects of nerve stimulation on the electrical and mechanical activity of the smooth muscle of the proximal renal pelvis of the guinea-pig were investigated using standard tension and microelectrode recording techniques. Spontaneous action potentials were deemed to have been recorded from three cell types. (1) “pacemaker” cells (9 of >120) had membrane potentials (MPs) of -42.1±2.9 mV and fired action potentials of a simple waveform; (2) “driven” cells (>100) had more stable MPs of -56.1±1.2 mV (n=36) and more complex “ureter-like” action potentials; (3) the remaining cells had MPs of -45.5±1.7 mV (n=15) and action potentials with a waveform “intermediate” to groups (1) and (2). Nifedipine (0.1–1 μM) and Cd²⁺ (0.1–1 mM) blocked all spontaneous action potential discharge and depolarized the membrane to near -40 mV. Intramural nerve stimulation (10–50 Hz for 1–10 s) increased both the …