Maitotoxin (MTX), a potent cytolytic agent, activates Ca(2+) entry via nonselective cation channels in virtually all types of cells. The identity of the channels involved and the biochemical events leading to cell lysis remain unknown. In the present study, the effect of MTX on plasmalemmal permeability of human skin fibroblasts was examined. MTX produced a time- and concentration-dependent increase in cytosolic free Ca(2+) concentration that depended on extracellular Ca(2+) and was relatively insensitive to blockade by extracellular lanthanides. MTX also produced a time- and concentration-dependent increase in plasmalemma permeability to larger molecules as indicated by 1) uptake of ethidium (314 Da), 2) uptake of YO-PRO-1 (375 Da), 3) release of intracellular fura 2 (636 Da), 4) uptake of POPO-3 (715 Da), and, ultimately, 5) release of lactate dehydrogenase (relative molecular weight of 140,000). At the single cell level, uptake of YO-PRO-1 correlated in time with the appearance of large MTX-induced membrane currents carried by the organic cation, N-methyl-D-glucamine (167 Da). Thus MTX initially activates Ca(2+)-permeable cation channels and later induces the formation of large pores. These effects of MTX on plasmalemmal permeability are similar to those seen on activation of P2Z/P2X(7) receptors in a variety of cell types, raising the intriguing possibility that MTX and P2Z/P2X(7) receptor stimulation activate a common cytolytic pore.