The previous observation that hysterectomy of rats results in increased pituitary nucleic acid turnover was followed up by studies on pituitary function in hysterectomized adult female rats with or without treatment with charcoal-treated crude uterine aqueous extracts. Hysterectomy had no effect on the pituitary or plasma LH and FSH concentrations. Administration of crude uterine extracts diminished the post-castrational rise of the pituitary FSH content without affecting the plasma FSH or LH concentrations. Hysterectomy decreased body weight but increased adrenal weight of ovariectomized animals. However, the body weight and pituitary weight of both the intact and ovariectomized animals increased significantly following 14 days of treatment with uterine extracts. The ovarian weight of the intact rats was reduced by this treatment. The pituitary nucleic acid content of castrated but oestradiol-substituted rats was reduced by hysterectomy. Plasma corticosterone concentrations were not affected by the above mentioned treatments. Uterine extracts inhibited the basal and GnRH induced LH release from isolated pituitary cells in culture. The effects of hysterectomy and/or administration of crude uterine aqueous extracts show that the uterus contains biologically active, non-steroidal substances which affect pituitary function of adult female rats.