Abstract: 4-Aminopyridine(4-AP) has been shown to induce histamine release fromisolated peritoneal mast cells(PMC)in mice and rats. In the presence of extracellular calcium atnormal level(0.9 mmol·L^-1)histamine release induced by 4-AP(13. 6 mmol·L^-1) from mice PMCwas 33.0%±4.6%,while at low calcium concentration (0.5 mmol·L^-1) and in calcium-freemedium this parameter decreased to 25.5%±4.2%and 16.3%±3.7%respectively.Histaminerelease in response to 4-AP(10 mmol·L^-1)from rat PMC was 39.1%±6.7%(0.9 mmol·L^-1calcium),while at low calcium concentration(0. 5 mmol·L^-1) and in calcium-free medium thisparameter decreased to 29.3%±4.7%and 20.2%±2.9% respectively. Results of statisticalanalysis indicate that 4-AP induced histamine release is related to Ca²⁺ concentration. When rat PMCwere preincubated in calcium-free medium with EDTA 0.1 mmol·L^-1 for 180 min　4-AP inducedhistamine release was 13.8%±1.6%.This shows that 4-AP also elicited mobilization of endogenous calcium stores in mast cells.The mechanism of 4-AP　induced histamine release was discussed.