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Extracellular activity was recorded from tuberoinfundibular neurones in 28 urethane-anaesthetized pro-oestrous rats. In each rat the electrical activity of one antidromically identified neurone was analysed before and during electrical stimulation in the preoptic/anterior hypothalamic areas (PO/AH). This stimulation (50 Hz, 30 s on and 30 s off for 15 min) resulted in significant increases in plasma concentrations of prolactin (from 46±11 (S.E.M.) ng/ml before stimulation to 86 ±17 ng/ml 20 min later; P<0·02) and LH (27 ± 6 to 48±11 ng/ml; P<0·01). A substantial proportion (11 out of 28) of the tuberoinfundibular neurones was inhibited by PO/AH stimulation. If such cells are directly involved with the secretion of anterior pituitary hormones it is probable that they synthesize and secrete inhibitory factors. We have suggested that cells inhibited by stimulation of the PO/AH are possibly the neurones which secrete prolactin inhibitory factor. The excited cells (eight out of 28) gave complex and variable responses to hypothalamic stimulation and were unable to follow applied stimuli at a ratio of more than one extra action potential for every ten stimulus pulses. We propose that this may explain why it is necessary to stimulate the PO/AH at ten times the frequency required for ovulation to occur following stimulation in the median eminence in anaesthetized pro-oestrous rats.