A previous study has suggested that a G to A base change at position 169 of the GHRH-receptor gene in human somatotrophinomas is a mutation and confers hypersensitivity to GHRH. The alternative base converts codon 57 from GCG to AGC, resulting in replacement of alanine (Ala) with threonine (Thr). In the present study, two of five human GH-secreting somatotrophinomas were found to possess the codon 57 AGC sequence. The GCG allele was also detected, indicating heterozygosity. However, the patients' normal blood-derived DNA also yielded the same sequence pattern, indicating that the Ala double right arrow Thr amino acid change is a normal polymorphism, and not a somatic mutation. Nevertheless, in vitro, the tumors possessing the Ala double right arrow Thr amino acid change responded very strongly to GHRH in terms of cAMP formation, being increased 40- and 200-fold, in comparison to the a-fold increases by tumors without the alternative GHRH-receptor sequence. Likewise, the in vitro response of GH secretion to GHRH was elevated. One of the two tumors with the alternative Thr residue, and the highest responder to GHRH, possessed a gsp mutation, despite the fact that these defects are thought to reduce responsiveness to GHRH. These results fail to confirm that the GCG double right arrow AGC at codon 57 of the GHRH-receptor gene is a mutation, but do support the concept that the alternative form with Thr confers increased sensitivity to GHRH.