The activity of enzymes inactivating oxytocin in the hypothalamus changes with stimuli known to release gonadotrophins and may be related to luteinizing hormone-releasing factor (LH-RF) release (Frith & Hooper 1971a,b; Griffiths & Hooper 1972). In the work presented here, enzyme activity was determined following neonatal testosterone treatment to assess any changes in the hypothalamic control of LH secretion. Supernatant and particulate fraction activities in female rats were depressed to male levels following injection on day 3; supernatant activity after injection on day 10 was also slightly decreased. These findings suggest that masculinization of the mechanisms controlling LH secretion in the hypothalamus, produced by neonatal testosterone, is caused by changes in LH-RF metabolism and that these peptidases in the rat hypothalamus are responsible for this metabolism.