Analogues of the potent alpha 2-adrenoceptor antagonist (8aR,12aS,13aS)-5,8,8a,9,10,11,12,12a,13,13a- decahydro-3-methoxy-12-(methylsulfonyl)- 6H-isoquino[2,1-g][1,6]naphthyridine (1b) were prepared and evaluated for alpha 1- and alpha 2-adrenoceptor affinity. Affinity for alpha 2-adrenoceptors was assessed by displacement of [3H]yohimbine from rat cerebral cortical membranes and although 1b and close structural analogues demonstrated high affinity, none were selective for the alpha 2A or alpha 2B subtypes reputedly present in this tissue. All of the high affinity alpha 2-adrenoceptor ligands were, however, selective with respect to [3H]prazosin (alpha 1) binding. Affinity for [3H]yohimbine-labeled alpha 2-adrenoceptors was found to be highly dependent on the stereochemistry of the tetracyclic system. The 8a beta,12a alpha,13a alpha diastereomer of 1 (56) had moderate affinity for alpha 2-adrenoceptors while the 8a beta,12a beta,13a alpha diastereomer (55) had very low affinity. The affinity and selectivity of these agents for alpha 2-adrenoceptors was found to correspond to that observed for several isomeric yohimbine analogues which have similar relative and absolute stereochemistries. Deviation from the structure of 1 by opening the B ring, changing the position of the sulfonamide nitrogen, or changing the attachment of the D ring led to a dramatic decrease in alpha 2-adrenoceptor affinity. High binding affinity was found to correlate with functional antagonism in the guinea pig ileum. The reversal of clonidine-induced mydriasis in the rat was used to assess bioavailability and indicated that 1b was a potent alpha 2-adrenoceptor antagonist in vivo.