Aminopyridine (4-AP) has been shown to be able to induce histamine release from rat peritoneal mast cells(PMC) .At physiological K⁺concentration(5.4 mmol/L) histamine release elicited by 4-AP(10 mmol/L) from rat PMC was 36.7%±4.9%,while at high K⁺concentration (150 mmol/L) and low K⁺concentration(2.7 mmol/L) this parameter decreased to 14.0% ±3.0 % and 8.2%±2.1% respectively.After preincubation with calcium-free medium contai ning EDTA (0.1 mmol/L) for 180 min,4-AP-induced histamine release from rat PMC was 13.9%±1.9%and K⁺ channel opener diazoxide did not inhibit any histamine release by 4-AP.The results indicate that either high K⁺ or low K⁺ could inhibit histamine release in response to 4-AP from rat PMC and that diazoxide failed to prevent the mobilization of endogenous calcium stores in PMC by 4-AP. The results also provide new evidence of the presence of K⁺ channels in mast cells.