Groups of 10 hemilobes of rat neurohypophyses were stimulated for 10 min in a medium containing 56 mm K+ or by application of a field current (20 Hz, 2 msec, 40 mA). Both stimulations resulted in a 5 to 8 fold increase in the release of vasopressin which was inhibited (in a dose-related manner) by addition to the medium of various blockers of calcium transport: LaCl3 (4 × 10−4, 4 × 10−3 m/l), prenylamine (2 × 10−5, 1 × 10−4 m/l) and D600, a verapamil derivative (4 × 10−6, 1 × 10−5, 2 × 10−5 m/l). Increasing the Ca2+ concentration in the medium to 14 mm abolished the inhibitory effect of D600. D600 also inhibited vasopressin release caused by introducing Ca2+ into a Ca2+-free medium during continued electrical stimulation. Efflux of 45Ca2+ from neural hemilobes labelled during incubation in a 56 mm K+ containing medium was markedly increased during stimulation with a 56 mm K+ medium or electrical current. Lanthanum (4 × 10−3 m/) and D600 (2 × 10−5 m) inhibited this increased efflux as well as hormone release. When the hemilobes had been pre-labelled in a medium with 4.8 mm K+, stimulation during efflux, by 56 mm K+ or electrical current, caused hormone release comparable to that seen after labelling in 56 mm K+, but with neither type of stimulation was there any significant increase in 45Ca2+ efflux. These findings would appear to be in agreement with a hypothesis that very small amounts of Ca2+ enter the endings of the neurosecretory neurons on stimulation (thereafter possibly releasing intracellular calcium) and that calcium entry is essential in inducing hormone release. The uptake of calcium over the neurosecretory nerve membrane apparently has several of the characteristics shown for calcium entry on stimulation of the squid axon.