Abstract： AIM　To investigate the signal transduction mechanism of histamine H3 r eceptor. METHODS　The adrenocorticotropic hormone (ACTH) levels of supernatants on AtT- 20 cells from the pituitary gland tumor were measured by radioimmunoassa y at the given time after histamine agonists were administrated and the effects o f R-(α)-methylhistamine on the cells proliferation were observed. RES ULTS　The H3 receptor specific agonist, R-(α)-methylhistamine incre ased the release of A CT H in time-dependent manner. It increased significantly after administrated r e agents 8 h which reached 1920 μg*L-1 compared to the control group 7 80 μg*L-1 . while the H1 receptor agonist 2-methylhistamine and the H2 agonist impro midine were significantly less potent. Furthermore, this response was blocked by thiop eramide, an H3 receptor specific antagonist, but not by the H1 and H2 anta gonist chlorpheniramine and cimetidine. R-(α)- methylhistamine had no significant i nf luence on cell proliferation within 24 h. Pretreatment with N-ethylmaleimide (N EM ) could abolish the effects of R-(α)-methylhistamine on the release of ACTH. CO NCLUSION　Specific activation of H3 receptor could evoke the excitatio n-secreti on coupling process, and G protein might be involved in the signal transdu ction.