The aim of this study was to correlate dopamine receptors and D(2) isoform expression with the cabergoline effect on alpha-subunit secretion in vitro and tumor mass in vivo in clinically nonfunctioning pituitary tumors. Eighteen patients were subjected to neurosurgery, and a tumor sample was used for dopamine receptor and D(2) isoform expression evaluation by RT-PCR and the in vitro functional studies. After neurosurgery, nine of 18 patients with persistent tumor were treated with cabergoline and tumor mass was evaluated before and after 1 yr treatment. D(2) receptor was expressed in 67% of cases. D(2long) was found in 50%, D(2short) in 17%, and both D(2) isoforms in 33% of cases. D(4) receptor was also expressed in 17% of cases. The in vitro inhibition of alpha-subunit concentration was found in 56% of cases and was associated with D(2) expression (chi(2) = 5.6; P < 0.05). After 1 yr of cabergoline treatment, tumor shrinkage was evident in 56% of patients and was associated with D(2) expression (chi(2) = 5.6; P < 0.05). The expression of D(2short) rather than D(2long) isoform is associated with the most favorable response of the tumor to cabergoline treatment. In conclusion, this study demonstrates D(2) receptor expression and function in nearly 70% of cases, suggesting a role of this drug in the treatment schedule of clinically nonfunctioning pituitary tumors.