Neuronal nicotinic acetylcholine receptors subserve predominantly modulatory roles in the brain, making them attractive therapeutic targets. Natural products provide key leads in the quest for nicotinic receptor subtype-selective compounds. Cytisine, found in Leguminosae spp., binds with high affinity to alpha 4 beta 2* nicotinic receptors. We have compared the effect of C3 and C5 halogenation of cytisine and methylcytisine (MCy) on their interaction with native rat nicotinic receptors. 3-Bromocytisine (3-BrCy) and 3-iodocytisine (3-ICy) exhibited increased binding affinity (especially at alpha 7 nicotinic receptors; K-1 similar to 0.1 mu M) and functional potency, whereas C5-halogenation was detrimental. 3-BrCy and 3-ICy were more potent than cytisine at evoking [H-3]dopamine release from striatal slices (EC50 similar to 11 nM), [H-3]noradrenaline release from hippocampal slices (EC50 similar to 250 nM), increases in intracellular Ca2+ in PC12 cells and inward currents in Xenopus oocytes expressing human alpha 3 beta 4 4 nicotinic receptor (EC50 similar to 2 mu M). These compounds were also more efficacious than cytisine. C3-halogenation of cytisine is proposed to stabilize the open conformation of the nicotinic receptor but does not enhance subtype selectivity.