The expression of two somatostatin receptor subtypes, SSTR1 and SSTR2, was studied in 27 pituitary tumors and 1 chronic lymphocytic leukemia pituitary infiltrate. Solution hybridization techniques were used for RNA analysis. SSTR1 and SSTR2 were, respectively, expressed in 3 of 7 and 9 of 10 GH-secreting tumors, 1 of 9 and 5 of 9 nonfunctioning tumors, 4 of 5 and 0 of 5 of prolactinomas, and 1 of 3 and 0 of 3 ACTH-secreting tumors. The chronic lymphocytic leukemia infiltrate expressed the SSTR2 subtype. No correlation was detected among tumor size, level of hormonal hypersecretion, and somatostatin receptor expression status. Two acromegalic patients who responded to octreotide therapy exclusively expressed the SSTR2 subtype in their tumors. The results indicate that two SSTR subtypes are heterogeneously expressed in different pituitary adenoma cell types.