We studied the effects of preoperative administration of histamine (H1 and H2) receptor blockers on hemodynamic changes in two groups of patients having operation for hydatid cyst. Patients in group 1 received no antihistaminics, whereas those in group 2 were pretreated with both H1 (diphenhydramine) and H2 (cimetidine) receptor blockers. Even though there were significant hemodynamic responses associated with spillage of hydatid cyst contents in both groups, the hemodynamic changes were significantly less in the patients who had received histamine receptor blockers. We conclude that the preoperative administration of H1 and H2 receptor blockers to patients having hydatid cyst surgery is beneficial.