The role of hyperinsulinaemia in neuroendocrine abnormalities in polycystic ovarian syndrome (PCOS) is controversial. The present study applied frequent blood sampling to assess the response of LH to metformin treatment in insulin-resistant women with PCOS. Thirteen predominantly overweight women with PCOS were studied before and after treatment with 1.5 g/day metformin for 3 months. Serum LH and testosterone were measured every 10 min for 10 h; LH was measured for an additional 2 h after gonadotrophin-releasing hormone (GnRH) administration. LH pulses were characterized by cluster analysis, secretory LH episodes by a deconvolution procedure, and synchronicity of paired LH-testosterone concentrations by lag-specific cross-correlation. After treatment, basal LH concentrations, amplitude of LH pulses, LH secretory amplitude, response to exogenous GnRH, and basal testosterone concentrations significantly decreased in seven patients, whereas in the remaining women these parameters remained unaltered. Before treatment, decreased coordinate LH and testosterone release was manifested by all patients; metformin treatment led to re-establishment of the feed-back control of testosterone on LH secretory rates by -20 to 0 min. Treatment did not modify the glucose:insulin ratio or serum insulin concentrations. In conclusion, administration of metformin allowed the identification of two subsets of PCOS women in whom neuroendocrine abnormalities may improve independently of the presence of insulin resistance or hyperinsulinaemia.