Abstract. Pre-operative bilateral simultaneous inferior petrosal sinus sampling with assessment of ACTH levels in the left and right sinuses and the periphery was performed in 9 patients with pituitary dependent Cushing's disease who were subsequently found at surgery to have basophil microadenomata. The novel observation of this study was the pattern of secretion of other pituitary hormones so that significant inter-sinus gradients ≥ 1.4:1 were seen for β-endorphin (2.8 ± 1.3, mean ± sem), PRL (4.2 ± 1.3) and GH (6.9 ± 2.4) as well as for ACTH (5.1 ± 1.1). There was no inter-sinus gradient for LH, FSH and TSH. In these 9 patients with adenomata, the correlations between the inter-sinus gradients for ACTH and β-endorphin were r = 0.95 (P <0.01), ACTH and PRL r = 0.90 (P < 0.01) and for ACTH and GH r = 0.89 (P <0.05). This close association between the gradients for ACTH and other anterior pituitary hormones could be due either to cosecretion of β-endorphin, PRL and GH by the ACTH-producing pituitary adenomata or to a paracrine effect of β-endorphin from the tumours on adjacent pituitary tissue. By reflecting the central pituitary hormone milieu, petrosal sinus sampling can give information about pituitary function unobtainable from peripheral hormone levels.