The expression of somatostatin receptor (SSTR) subtypes and relative abundance of SSTR2 mRNA were examined in 18 pituitary adenomas using the reverse transcription-polymerase chain reaction (RT-PCR) method. SSTR1 and SSTR2 were expressed in all pituitary adenomas examined. Six of 9 somatotroph adenomas, 1 of 4 lactotroph adenomas and 1 of 2 thyrotroph adenomas also expressed SSTR5. SSTR3 and SSTR4 mRNAs were detected in 1 and 2 cases of somatotroph adenoma, respectively. SSTR2 mRNA expression was quantified by comparison with the PCR cycle-dependent amplification of beta-actin or cyclophilin. The relative abundance of SSTR2 mRNA varied greatly among adenomas with more than a 1000-fold difference. SSTR2 mRNAs in lactotroph adenomas were less abundant (P < 0.01) than those in somatotroph adenomas. No significant correlation was found between the relative abundance of SSTR2 mRNA levels and GH sensitivity to octreotide administration. However, one of the thyrotroph adenomas exhibited marked shrinkage in tumor size after octreotide therapy, in which SSTR2 mRNA was the most abundant among the adenomas examined. GH sensitivity to octreotide was not significantly different between SSTR5 mRNA positive and negative adenomas. In conclusion, SSTR2 mRNA levels varied greatly among pituitary adenomas but were not correlated with GH sensitivity to octreotide. Further investigations of functional SSTR subtype proteins and of postreceptor signal transductions are required to clarify the molecular mechanisms of octreotide action.