Large and rapid changes in light scattering accompany secretion from nerve terminals of the mammalian neurohypophysis (posterior pituitary). In the mouse, these intrinsic optical signals are intimately related to the arrival of the action potential E-wave and the release of arginine vasopressin and oxytocin (S-wave). Here we have used a high bandwidth atomic force microscope to demonstrate that these light-scattering signals are associated with changes in terminal volume that are detected as nanometer-scale movements of a cantilever positioned on top of the neurohypophysis. The most rapid mechanical response ("spike"), having a duration shorter than the action potential but comparable to that of the E-wave, represents a transient increase in terminal volume due to water movement associated with Na(+)-influx. The slower mechanical event ("dip"), on the other hand, depends upon Ca(2+)-entry as well as on intraterminal Ca(2+)-transients and, analogously to the S-wave, seems to monitor events associated with secretion.