Growth hormone (GH) and prolactin (PRL) responses after TRH administration were studied in 31 women presenting with the clinical, biochemical and ultrasonographic characteristics of the polycystic ovarian (PCO) syndrome; their results were compared with those of 20 normally menstruating women investigated during the early follicular phase of the cycle. Based on the GH responses two PCO subgroups were observed: (a) nonresponders (n = 16) who showed △max GH responses (0.7 ± 0.27 ng/ml, x ± SE) similar to those of the normals (0.97 ± 0.20 ng/ml), and (b) responders (n = 15), 48.4% of the PCO patients who showed a paradoxical increase in GH levels (△max GH, 18.0 ± 1.96 ng/ml) following thyrotropin-releasing hormone (TRH) administration significantly higher than those observed either in nonresponder PCO patients or in normals. Furthermore, basal GH levels were found to be significantly higher in the responder PCO subgroup (5.65 ± 0.75 ng/ml) compared to either nonresponders (1.58 ± 0.21 ng/ml) or normals (1.8 ± 0.18 ng/ml). However, no correlation was found between basal GH levels and △max GH responses observed. Additionally, basal PRL and △max PRL levels following TRH administration did not differ either between the two PCO subgroups or those observed in normal controls. Δ<sub>4</sub>A, T and E<sub>2</sub> levels were similar between the two PCO subgroups. No correlation was found between the △max GH responses to △max PRL or the post-luteinizing hormone-releasing hormone stimulation test △max luteinizing hormone:follicle-stimulating hormone ratio observed or to steroid levels. These data are suggestive of an abnormal responsiveness of somatotrophs to TRH stimulation in 48.4% of PCO patients which could not be correlated either with an abnormality of lactotrophs or with a difference of the androgen and E<sub>2</sub> levels.