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Maitotoxin is a potent marine poison that mobilizes calcium in most vertebrate cell types and accelerates secretion from anterior pituitary cells. It is not known whether voltage-sensitive calcium channels or other mechanisms initiate the effects of maitotoxin on anterior pituitary cells. Changes in intracellular Ca²⁺ levels may also be achieved by releasing internal calcium stores via inositol trisphosphate (InsP₃). Indeed, maitotoxin rapidly increased inositol phosphate accumulation in a concentration-dependent manner. Calcium channel antagonists such as nifedipine and verapamil did not block this response nor did calcium-mobilizing agents (BAYk8644, A23187 [GenBank] ) mimic this effect. These data suggest that the mechanism by which maitotoxin acts at the pituitary may include the activation of an enzyme that produces the calcium-mobilizing signal InsP₃.