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The effects of urethane or sodium pentobarbitone anaesthesia on the feedback effects of oestradiol or progesterone on gonadotrophin secretion in the ovariectomized guinea-pig have been investigated. In the control experiments in which no steroids or vehicles were given the concentration of LH and FSH in samples of peripheral blood collected at intervals of 15 min varied in a random episodic manner. The mean level of LH was significantly greater in sodium pentobarbitone- and urethane-anaesthetized animals when compared with conscious animals, and in the conscious animals there was a progressive fall in mean LH level during the course of serial sampling. This effect was not observed in anaesthetized animals.

Oestradiol benzoate (2 µg s.c.) inhibited LH secretion in conscious animals and in those anaesthetized with sodium pentobarbitone, but not in urethane-anaesthetized guinea-pigs. Progesterone (200 µg s.c.) progressively depressed plasma LH levels in conscious and urethane-anaesthetized animals, but not in guinea-pigs anaesthetized with sodium pentobarbitone. The effect of anaesthesia was the converse of that observed with oestradiol treatment.

Significant changes in FSH secretion were not observed under any experimental conditions.

These findings point to the existence of a mechanism in spayed guinea-pigs that restrains LH secretion, the action of which is reduced by anaesthesia and modified by oestradiol and progesterone.