1. Binding of the novel radioligand (3)H-2-(2-dimethylaminomethyl-phenylsulphanyl)-5-methyl-phenylamine ((3)H-MADAM) to the serotonin transporter (SERT) was used to characterise a range of selective serotonin re-uptake inhibitors (SSRIs) in vitro and in vivo. 2. (3)H-MADAM bound with high affinity in a saturable manner to both human SERT expressed in CHO cells (K(d)=0.20 nm (pK(d)=9.74+/-0.12), B(max)=35+/-4 fmol mg(-1) protein) and mouse cerebral cortex membranes (K(d)=0.21 nm (pK(d)=9.66+/-0.10), B(max)=50+/-24 fmol mg(-1) protein). 3. Binding of (3)H-MADAM was highly selective for SERT in vitro as demonstrated by the in vitro profile of MADAM tested at 75 different receptors, ion channels and transporters. This was further substantiated by the pharmacological profile of the binding. Hence, the binding of (3)H-MADAM was potently inhibited by SSRIs but not by selective inhibitors of noradrenaline transport and dopamine transport. Likewise, a 5-HT(2A/2C) receptor antagonist did not inhibit (3)H-MADAM binding. 4. (3)H-MADAM binding in vivo was inhibited only by compounds which also inhibited the binding of (3)H-MADAM in vitro (the SSRIs, mixed SERT/noradrenaline transport inhibitors and clomipramine), confirming the selectivity of (3)H-MADAM for SERT also in vivo. Moreover, compounds effective in inhibiting (3)H-MADAM binding were the only ones found to be active in the mouse 5-HTP potentiation test confirming the model as a behavioural correlate to in vivo 5-HT uptake. 5. Finally, it was found that a SERT occupancy of 85-95% was necessary to produce 50% of the maximum behavioural response (ED(50)).