Inappropriate stimulation of growth hormone (hGH)-secretion with TRH in acromegalic subjects has been shown previously, whereas prolactin (hPRL) secretion was reported to be blunted in active acromegaly. In this study TRH induced hGH and hPRL secretion was investigated in 23 active (mean hGH level: 68.5 +/- 19.9 ng/ml; +/- SE) and 15 inactive acromegalics (mean hGH level: 2.3 +/- 0.4 ng/ml; +/- SE). Fourteen of the active acromegalics showed a significant increase up to more than double of the basal hGH level after 200 mug TRH while in only one of the inactive acromegalics an inappropriate rise of hGH was induced by TRH. Basal hPRL levels were elevated in 9 and within normal range in 14 of the patients with active acromegaly. Except in 3 patients who had normal basal hPRL levels and in one patient with excessively elevated hPRL and hGH levels due to an hPRL and hGH producing adenoma, all patients had normal hPRL responses to TRH. In those with inactive acromegaly, 9 had hPRL levels below normal and only , out of the 15 patients showed a normal rise of hPRL after TRH. The patients who showed no hPRL response to TRH demonstrated also other signs of pituitary insufficiency due to the operative procedure. In contrast to a previous report these findings demonstrate that normal or enhanced hPRL secretion is found in active acromegaly. The inappropriate rise of hGH after TRH is compatible with a loss of specificity of the receptor for GRH of the adenoma cell and can be found also in patients with normal basal hGH levels after treatment suggesting that remaining adenoma tissue is present in the pituitary fossa.