ω-Agatoxin TK, a peptidyl toxin of the venom of Agelenopsis aperta, is a potent and selective P/Q type Ca2+ channel blocker. ω-Agatoxin TK inhibits the high K+ depolarisation-induced rise in internal Ca2+ in cerebral isolated nerve endings with an IC50 of of 60 nM. ω-Agatoxin TK has no effect on L-type, N-type, or T-type calcium channels.