The isolated rat fundus preparation was used to determine the serotonin (5-HT) receptor affinities (pA2 values) of harmaline, hatmine and several other related B-carbolines. Replacement of a 7methoxy group by a hydroxy group decreases affinity; the 6-methoxy B-carbolines possess two to three times the affinity of their 7-methoxy isomers. A new model is proposed for the interaction of these B-carbolines with 5-HT receptors.