Besides apamin, the structurally related MCD peptide (mast cell degranulating peptide; peptide 401) is another centrally acting peptide from bee venom. In contrast to apamin, it is hardly neurotoxic upon intravenous injection in mice. Following intraventricular injection, as little as 0.3 microgram/animal produce convulsions and respiratory arrest in mice. The clinical picture differs from that elicited by apamin, and apamin is about 10 times more potent than MCD peptide when given intraventricularly. Apamin and MCD peptide injected into the spinal cord of rats in nanogram amounts, produce circumscript hyperexcitation lasting more than one day, however with complete recovery following sublethal doses. Local apamin poisoning differs from local tetanus (elicited by the same way) by its faster time course.