1. The anti-thyroid drug, propylthiouracil (0.05% w/v in drinking water) was administered to rats for 3-4 weeks. This treatment decreased the rate at which rats gained body weight but did not affect the weight of the vasa deferentia. Circulating levels of thyroxine and triiodothyronine were markedly decreased.2. The potencies of postjunctional a-adrenoceptor agonists, 1 -noradrenaline and 1-phenylephrine, in eliciting contraction of the epididymal end of the vas deferens were unaffected by propylthiouracil treatment. In contrast, the potency of the 0-adrenoceptor agonist, 1 -isoprenaline, in inhibiting field stimulationinduced twitches of the prostatic end of the vas deferens was decreased in tissues from propylthiouracil-treated rats. Thus hypothyroidism induced by propylthiouracil produced an apparent change in the properties of postjunctional P-adrenoceptors without a concomitant reciprocal change in the properties of postjunctional a-adrenoceptors.3. Treatment of rats with propylthiouracil led to a small increase in the concentration of noradrenaline in the vas deferens, and a concomitant small increase in the rate of accumulation of 3H-noradrenaline in in vitro experiments. Extraneuronal uptake of ' H-isoprenaline was unaffected by propylthiouracil treatment.
4.The potencies of the prejunctional a-adrenoceptor agonists, xylazine and 1 -noradrenalhe, in producing inhibition of twitches evoked by field stimulation of preparations of the prostatic end of the vas deferens, were similar in preparations from propylthiouracil-treated and control rats, although the slope of the log dose-response curve to xylazine was decreased in the former group. Thus unequivocal changes in the properties of prejunctional a-adrenoceptors did not accompany the other prejunctional effects of propylthiouracil upon endogenous noradrenaline levels and neuronal uptake of H-noradrenaline in this organ.