1. The effects of pretreatment with pertussis toxin (50-200 micrograms kg-1, i.p.) on the pre-junctional inhibitory actions of the alpha 2-adrenoreceptor agonist xylazine were examined by employing the pithed rat and rat isolated vas deferens. 2. In pithed rats, pertussis toxin attenuated the pressor response to xylazine, but did not alter the pre-junctional cardio-inhibitory actions of xylazine. In epididymal portions of rat vas deferens, pretreatment with pertussis toxin did not alter the pre-junctional inhibitory action of xylazine. 3. These data lend no support for the view that alpha 2-adrenoreceptor-mediated prejunctional inhibition involves inhibition of adenylate cyclase through the Ni regulatory protein, at least in the tissues examined.