Electrophysiological studies were carried out on long term cultured brainstem tissue taken from neonatal rats with the object of investigating mechanisms underlying respiratory rhythm generation. The preparations were derived from 360 microns thick horizontal medullary slices which were explanted into a chemically defined nutrient medium and which remained organotypically intact for ca. 1 month. In 44 of the 50 explants examined both periodic and aperiodic bioelectric activity was detected, the cycle length of the former ranging from 0.5 to 10 s (mean, 2.7 s) at a pH of 7.4 and bath temperature of 32 degrees C. Periodic activity could take several forms, but commonly consisted of regularly repeated, 100-300 ms long, depolarizing (D-) waves or sequences of inhibitory and/or excitatory postsynaptic potentials. Lowering the pH of the superfusate by lowering the bicarbonate concentration, increasing the pCO2 or adding H+ shortened the interval between periodic events, and increased both the amplitude and duration of the D-waves. The interval was also shortened when the bath temperature was increased (Q10: ca.2.5). The mean resting membrane potential of neurons exhibiting periodic activity was -49 mV (n = 62) and not significantly different from that of aperiodically discharging neurons either in the same preparations or in cultured explants from the neocortex. These observations suggest that brainstem cultures constitute a useful 'model' system for studying pH-dependent rhythm generation in small neuronal networks of the medulla.