The acrosome reaction (AR) is a Ca2+-dependent event required for sperm to fertilize the egg. The activation of T-type voltage-gated Ca2+ channels plays a key role in the induction of this process. This report describes the actions of two toxins from the scorpion Parabuthus granulatus named kurtoxin-like I and II (KLI and KLII respectively) on sperm Ca2+ channels. Both toxins decrease T-type Ca2+ channel activity in mouse spermatogenic cells and inhibit the AR in mature sperm. Saturating concentrations of the toxins inhibited at most similar to70%, of the whole-cell Ca2+ current, suggesting the presence of a toxin-resistant component. In addition, both toxins inhibited similar to60% of the AR, which is consistent with the participation of T-type Ca2+ channels in the sperm AR, (C) 2002 Elsevier Science (USA). All rights reserved.