The gonadotrophin responses to the negative and positive feedback effects of orally administered ethinyl oestradiol in 12 women with minimal endometriosis were compared with those in six normal women of proven fertility. Basal concentrations of LH, FSH and oestradiol did not differ between the two groups. LH and FSH concentration declined within 12-24 h of starting treatment and there was no difference between minimum values, which occurred in both groups after 12-48 h. In five women with minimal endometriosis, LH concentrations subsequently increased above basal values to maximum values indistinguishable in magnitude and timing from those in the control group. An exaggerated LH response to oestrogen positive feedback occurred in one subject. In the remaining six women LH concentrations increased above levels which could be attributed to episodic variation in basal LH secretion but maximum LH values were below control limits. There was no relationship between previously documented progesterone secretion nor LH concentrations during the early luteal phase of the cycle and the LH response to oestrogen positive feedback. These data suggest that minor abnormalities of hypothalamopituitary function may contribute to infertility associated with minimal endometriosis.