BACKGROUND Higher vasopressin (AVP) levels have been found in the inferior petrosal sinus ipsilateral to the ACTH‐secreting adenoma than in the contralateral one, suggesting a potential pathogenetic role of AVP in Cushing's disease.DESIGN In order to investigate AVP release, plasma ACTH and AVP concentrations were assayed in the inferior petrosal sinuses and in the peripheral blood before and after CRH stimulation.PATIENTS Twenty patients with Cushing's disease and 12 with other pituitary diseases were subjected to simultaneous and bilateral inferior petrosal sinus sampling for diagnostic purposes. Ten healthy sex and age‐matched subjects served as control for peripheral AVP values.MEASUREMENTS Plasma ACTH concentrations were measured by RIA using commercial kits. Plasma AVP concentrations were assayed by RIA in acetone extracts of 1–2 ml plasma.RESULTS Plasma AVP levels in the inferior petrosal sinuses were significantly higher in Cushing's disease than in patients with other pituitary diseases (P<0.05) and in both groups AVP levels were higher in the inferior petrosal sinuses than in the peripheral blood (P<0.01). In Cushing's disease, ACTH, but not AVP levels, were higher in the inferior petrosal sinus ipsilateral to the adenoma than in the contralateral one (P<0.01). Seven patients showed a significant ACTH and AVP increase (greater than 50% of baseline) after CRH stimulation in the inferior petrosal sinus ipsilateral to the adenoma. Conversely, no change was found in AVP levels in the remaining 13 patients. When AVP values were analysed in relation to surgical cure, higher inferior petrosal sinus levels (P<0.05) were found in 6 patients with poor outcome: 4 of these patients had significantly decreased plasma AVP concentrations (by 32–43% of baseline) after CRH bolus. Peripheral AVP levels were similar in healthy subjects and patients with Cushing's disease whereas they were significantly reduced in patients with other pituitary diseases (P<0.002).CONCLUSIONS The results of this study show that patients with Cushing's disease and poor surgical outcome had the highest AVP levels in our series. CRH administration caused different effects on AVP levels: it increased them in 35% of patients whereas there was no response in the remaining patients. On the basis of these findings, it is hypothesized that AVP might be involved in the persistence of ACTH hypersecretion in a subset of patients poorly