1. In this study the secretory response of individual bovine adrenal chromaffin cells was monitored using amperometric carbon-fibre microelectrodes. Cells were stimulated to secrete by exposure to 20-100 μs long tonehursts of ultrasonnd (2-4 x 10⁵ Pa; peak pressure at 1 MHz). 2. Three types of secretory responses were observed: an almost instantaneous response, a delayed release of catecholamines, or a series of 'burst-like' secretory bouts. 3. Fura-2 measurements of intracellular Ca²⁺ concentrations showed that the release of catecholamines was accompanied by an increase in the intracellular Ca²⁺ concentration. In the absence of extracellular Ca²⁺, secretory responses were not evoked showing that Ca²⁺ entry was necessary to elicit catecholamine release.