Benzotropine mesylate [132-17-2], promethazine [60-87-7], cyproheptadine [129-03-3], chlorpheniramine maleate [113-92-8], diphenhydramine [58-73-1], pyrilamine maleate [59-33-6], and tripelennamine [91-81-6] injected into squirrel monkeys increased responding under fixed interval schedules.The relative potencies of the drugs were similar to the histaminic H1 antagonist potencies and not to other possible mechanisms such as inhibition of dopamine uptake, serotonin antagonism, or antimuscarinic effects.