In the search for new 5-hydroxytryptamine (5-HT) receptor antagonists it was found that the compound (R)-(+)-2-[[[3-(morpholinomethyl)-2H-chromen-8-yl]oxy] methyl] morpholine methanesulfonate, (R)-25, is a selective rat 5-hydroxytryptamine1B (r5-HT1B) receptor antagonist. The binding profile showed a 13-fold preference for r5-HT1B (Ki = 47 +/- 5 nM; n = 3) vs bovine 5-HT1B (Ki = 630 nM; n = 1) receptors. The compound had very low affinity for other monoaminergic receptors examined. The r5-HT1B receptor antagonism was demonstrated by the potentiation of the K+-stimulated release of [3H]-5-HT from superfused rat brain slices in vitro, an effect that was antagonized by addition of 5-HT to the superfusion fluid. (R)-25 at 20 mg/kg sc enhanced the 5-HT turnover in four rat brain regions (hypothalamus, hippocampus, striatum, and frontal cortex) with about 40% measured as the 5-HTP accumulation after decarboxylase inhibition with 3-hydroxybenzylhydrazine. At 3 mg/kg sc (R)-25 produced a significant increase in the number of wet dog shakes in rats, a 5-HT2A/5-HT2C response that was abolished by depletion of 5-HT after pretreatment with the tryptophan hydroxylase inhibitor p-chlorophenylalanine. These observations show that (R)-25, by inhibiting terminal r5-HT1B autoreceptors, increases the 5-HT turnover and the synaptic concentration of 5-HT.