NAS-181 ((R)-(+)-2-(3-morpholinomethyl-2H-chromen-8-yl) oxymethyl-morpholine methanesulfonate) is a novel rat 5-hydroxytryptamine1B, (r5-HT1B) receptor antagonist with high selectivity. The in vivo effects of NAS-181 on 5-HT metabolism and synthesis in the rat brain were examined. 5-HT metabolism, measured as the ratio 5-hydroxyindoleacetic acid (5-HIAA)/5-HT, was dose-dependently increased in all four brain regions analysed (hypothalamus, hippocampus, frontal cortex and striatum) at doses 0.1 to 20 mg/kg s.c. NAS-181. The enhancement of 5-HT metabolism at the dose 20 mg/kg s.c. was maximal one hour after the injection and was still significant eight hours but not 24 hours after the injection. 5-HT synthesis rate measured as the accumulation of 5-hydroxytryptophan (5-HTP) after inhibition of the aromatic amino acid decarboxylase activity was also elevated by NAS-181 at doses 0.3 to 20 mg/kg s.c. NAS-181 competitively antagonised the decrease in 5-HT metabolism evoked by the r5-HT1B receptor agonist, anpirtoline, in hypothalamus, hippocampus and frontal cortex. Anpirtoline had no effect on 5-HT metabolism in striatum. However, anpirtoline antagonised the enhancement of 5-HT metabolism induced by NAS-181 in striatum. Combined treatment of rats with NAS-181 and the 5-HT1A receptor antagonist, WAY-100635, increased 5'-HT metabolism considerably more than when the compounds were given alone.