Alpha latrotoxin, purified from the venom of the black widow spider, is a high Mr (130000) protein devoid of detectable enzymatic activity. When applied to vertebrate nerve terminals (of the central as well as peripheral nervous systems) the toxin elicits massive release of neurotransmitters by stimulating the fusion of synaptic vesicles with the presynaptic membrane (exocytosis). Among non-neuronal systems, only the neurosecretory cell line PC12 is sensitive to alpha latrotoxin; all others investigated so far are insensitive. In order to act, alpha latrotoxin requires the presence of divalent cations in the medium. Ca2+ can be substituted by other divalent cations as Sr2+, Ba2+, Mn2+, Mg2+. However, with the last two the catecholamine release response is reduced in PC12 cells and synaptosomes. A specific, high affinity receptor of alpha latrotoxin exists in preparation sensitive to the toxin. This receptor has been purified and found to be a high Mr, integral membrane protein. In the frog neuromuscular junction the receptor is localized exclusively in the presynaptic membrane. Binding of alpha latrotoxin to the receptor in a Ca2+-containing incubation medium induces membrane depolarization (insensitive to tetrodotoxin), stimulation of Ca2+ influx (insensitive to verapamil) with consequent increase in the cytoplasmic free Ca2+ concentration and stimulation of phosphoinositide breakdown. In Ca2+ free medium depolarization is maintained, but free Ca2+ concentration does not rise after toxin application. In conclusion, alpha latrotoxin seems to act through a dual mechanism. The Ca2+-independent part of this mechanism may be mediated by the activation of protein kinase C, triggered by phosphoinositide metabolism. The relevance of these findings for presynaptic physiology is discussed.