Because phosphodiesterase (PDE) expression and activity are controlled by cAMP, we investigated whether activating mutations of Gsα gene that occur in human GH-secreting adenomas are associated with increased PDE activity. We studied 10 adenomas with wild-type Gsα (gsp−) and 8 with mutant Gsα (gsp+). Although, in the absence of PDE inhibitors, intracellular cAMP levels were similar in gsp+ e gsp− adenomas, the PDE blockade with 3-isobutyl-1-methylxanthine induced a marked increase in cAMP in all but one gsp+ adenoma (% increase: from 77 to 2900) and a slight rise in only 2 gsp−. Similar results were obtained with the PDE4 selective inhibitor 4-[3-(cyclopentyloxy)-4-methoxyphenyl]-2-pyrrolidinone. In vitro GH release was significantly higher in gsp+ than in gsp− adenomas (315 ± 158 vs. 82 ± 53 μg/well; P< 0.01), and PDE blockade caused a further increase in 3 of 5 gsp+ adenomas but not in gsp− tumors. By direct measurement, PDE activity was about 7-fold higher in gsp+ than in gsp− adenomas (320 ± 213 vs. 48 ± 23 pmol/min·mg protein; P < 0.05) and was largely 4-[3-(cyclopentyloxy)-4-methoxyphenyl]-2-pyrrolidinone sensitive. This study first demonstrates that activating mutations of the Gsα gene that naturally occur in pituitary adenomas is associated with an increased PDE activity that might, at least partially, counteract the constitutive activation of the cAMP-dependent pathway.