In the population of primary cultured rat chromaffin cells, over half exhibited spontaneous [Ca2+]i oscillations, whereas most others were induced to oscillate by low concentrations of bradykinin or KCl. [Ca2+]i spots were observed to pulsate in a defined cytoplasmic area (the oscillator). In silent cells those spots remained discrete, whereas in oscillating cells the [Ca2+]i increase expanded to occupy the entire cytoplasm. Alternation of these discrete and expanded events was observed in a few irregularly oscillating cells. Thapsigargin induced prompt blockade of both pulsations and oscillations and prevented recruitment of silent cells to oscillate. This indicates sarcoendoplasmic reticulum Ca(2+)-ATPase-type Ca2+ pump(s) to be crucial for the functioning of the oscillator. Effects of other treatments were variable, depending on the concomitant [Ca2+]i changes. Oscillations were blocked when EGTA or nitrendipine decreased Ca2+ influx and thus [Ca2+]i; they were also blocked when [Ca2+]i was markedly increased by excess KCl, bradykinin, or ryanodine. When in contrast the [Ca2+]i increases induced by the latter agents remained moderate, oscillations were stimulated. The rhythmic activity of rat chromaffin cells appears, therefore, to operate under a complex regulation that requires [Ca2+]i within an appropriate operative range and does not involve directly the ryanodine receptor but might rely on the activation of IP3 receptors.