A multihormonal response to CRH during inferior petrosal sinus sampling in patients with Cushing's disease has recently been described. Whether it reflects multihormonal secretion by the corticotropic adenoma, or secretion by non-tumorous adjacent cells via paracrine mechanisms remains debatable. We have compared the effect of CRH on ACTH, GH, PRL and TSH secretion during inferior petrosal sinus sampling with its effect on the in vitro secretion of the corticotropic adenoma after excision in one case of Cushing's disease. Before CRH injection in vivo results show significant central-peripheral gradients for all hormones but only ACTH lateralized to the side of the tumor. After CRH administration, the petrosal concentrations of all hormones increased preferentially on the side of the adenoma resulting in significant intersinus gradients: 8.1 for ACTH, 2.0 for GH, 1.8 for PRL and 1.5 for TSH. In vitro results: the adenoma cells were immunostainable for ACTH only. In culture, they secreted ACTH only. Addition of CRH to the culture induced a mean increase of 160% in ACTH secretion but GH, PRL and TSH remained undetectable. Our results favor the hypothesis that the multihormonal response to CRH seen during inferior petrosal sinus sampling in Cushing's disease reflects a paracrine stimulation of the adjacent non-tumorous pituitary cells by the corticotropic adenoma.