Changes in cytosolic calcium ion concentration ([Ca2+]i) have been implicated in a wide variety of cellular stimulus--transduction roles. In nerve cells, it is believed that electrical activity raises [Ca2+]i by allowing influx of Ca2+ through voltage-dependent channels in the surface membrane. Elevation of neuronal [Ca2+]i may also occur due to release of Ca2+ from intracellular storage sites. Transient increases in [Ca2+]i are thought to trigger neurotransmitter release, and to modulate axonal transport, energy metabolism and growth cone movement. Intracellular Ca2+ also appears to regulate membrane potassium channels and thereby to regulate electrical excitability. Although [Ca2+]i transients have been measured in a few giant invertebrate neurones, detection of such transients in a vertebrate neurone has not been previously reported. We have measured [Ca2+]i in bullfrog sympathetic neurones by photometry of a microinjected calcium indicator dye, arsenazo III (refs 14-16), and report here that action potentials and voltage-clamped depolarizations cause long-lasting increases in [Ca2+]i. Aslo, exposure to the drug theophylline can cause spontaneous periodic increases in [Ca2+]i. Comparisons of [Ca2+]i signals with simultaneous intracellular recordings of membrane potential suggest that the kinetics of the post-tetanic hyperpolarization (PTH) following a series of action potentials or the spontaneous hyperpolarizations induced by theophylline directly reflect the kinetics of the [Ca2+]i transient.