Mono-O-demethylation of several 2,5-dimethoxyphenalkylamines increases their affinity for the serotonin receptors of the isolated rat fundus preparation. In several instances, demethylation of methoxyphenalkylamines results in compounds which produce an antagonism which is not of a competitive nature. With respect to 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM), demethylation of the 2-methoxy group alters affinity in a manner which parallels that observed upon demethylation of 5-methoxy-N,N-dimethyltryptamine. Using a discriminative stimulus paradigm, behavioral studies with rats reveal that the 2-hydroxy analogue, but not the 5-hydroxy analogue, of DOM produces effects (interoceptive cues) similar to those produced by 5-methoxy-N,N-dimethyltryptamine.