The present pharmacological study was conducted to investigate a possible role of the brain histaminergic system in vocalization induced in guinea pig pups by maternal separation and isolation in an unfamiliar environment. The effects of drugs acting at histamine receptors were determined after intraperitoneal injection, comprising hydroxyzine and chlorpheniramine, both histamine H1 antagonists, and the H3 agonist, immepip. A range of psychoactive drugs known to be active in this paradigm was also tested for comparison. Hydroxyzine, 4.3 to 14.3 mg/kg, dose-dependently suppressed vocalization but neither chlorpheniramine, 2 to 16 mg/kg, nor immepip, 5 to 20 mg/kg, was active. All reference drugs, fluoxetine, 5 and 10 mg/kg, imipramine, 16 and 32 mg/kg, and chlordiazepoxide, 5 and 10 mg/kg, were shown to be active. The present data indicate that, consistent with known anxiolytic effects in man, the antihistamine hydroxyzine proved effective in suppressing maternal-separation-induced vocalization in guinea pig pups. However, in view of the lack of effect of either chlorpheniramine or immepip, it is proposed that additional nonhistaminergic effects are involved in the tranquilizing action of hydroxyzine.