Anaphylactic shock accidents after allergen exposure are frequent. After immunization with ovalbumin (OVA), a common dietary constituent, we evaluated the efficacy of pretreatment with histamine-receptor or serotonin-receptor blockers administered alone or in combination with a nitric oxide synthase inhibitor (L-NAME) on OVA-induced anaphylactic shock in Brown Norway rats. Animals were allocated to the following groups (n= 6 each): control (0.9% saline); diphenydramine (15 mg kg− 1); cimetidine (20 mg kg− 1); diphenydramine+ cimetidine; dihydroergotamine (50 μg kg− 1); diphenydramine+ cimetidine+ dihydroergotamine; L-NAME (100 mg/kg) alone or associated with diphenydramine, cimetidine, diphenydramine+ cimetidine, dihydroergotamine, or diphenydramine+ cimetidine+ dihydroergotamine. Mean arterial blood pressure (MABP), heart rate (HR), and survival time were monitored for 60 min following treatment. The shock was initiated with iv OVA. The MABP drop after iv OVA was worsened by diphenydramine and was modestly attenuated by cimetidine, dihydroergotamine, or both together. L-NAME potentiated slightly the effects of cimetidine and dihydroergotamine by lessening the initial MABP decrease, but this transient effect was not sufficient to prevent the final collapse or to improve survival time. Decreased vasodilatory (prostaglandins E2), increased vasoconstrictory (thromboxane B2) prostaglandins, and unchanged leukotriene C4 concentrations were contributory to the overall hemodynamic changes. Thus, the combined blockade of vasodilator mediators (histamine, serotonin, and nitric oxide) slowed the MABP drop in anaphylactic shock, but did not improve survival. More studies are needed to understand these discordant effects.