Electrical activity in non-neuronal cells can be induced by altering the membrane potential and eliciting action potentials. For example, hormones, nutrients and neurotransmitters act on excitable endocrine cells¹. In an attempt to correlate such electrical activity with regulation of cell activation, we report here direct measurements of cytosolic free Ca²⁺ changes coincident with action potentials. This was achieved by the powerful and novel combination of two complex techniques, the patch clamp² and micro-fluorimetry using fura 2 methodology^3,4. Changes in intracellular calcium concentration were monitored in single cells of the pituitary line GH₃B₆. We show that a single action potential leads to a marked transient increase in cytosolic free calcium. The size of these short-lived maxima is sufficient to evoke secretory activity. The striking kinetic features of these transients enabled us to identify oscillations in intracellular calcium concentration in unperturbed cells resulting from spontaneous action potentials, and hence provide an explanation for basal secretory activity. Somatostatin, an inhibitor of pituitary function⁵, abolishes the spontaneous spiking of free cytosolic Ca²⁺ which may explain its inhibitory effect on basal prolactin secretion. Our data therefore demonstrate that electrical activity can stimulate Ca²⁺-dependent functions in excitable non-neuronal cells.