The uterine neurotransmitter, noradren,line, was examined histochemically and fluorometrically in guinea-pigs during the estrous cycle and after pretreatment with sex steroids. In diestrous animals noradrenaline values were higher than in the estrous state. Treatment with estradiol or progesterone alone did not markedly influence the level of uterine adrenergic transmitter, whereas combined estradiol and progesterone administration caused a clear-cut reduction in uterine noradrenaline. The quantitative changes in transmitter level did not reflect the marked alterations in weight during the various experimental conditions, and even inverse noradrenaline-weight relationships were observed. The findings offer further support for the concept of a steroid-mediated influence on uterine neurotransmission, where the uterine adrenergic nerves constitute a separate target for endocrine control, distinct from the rest of the uterus.