The affinities of a series of 1-(2,5-dimethoxyphenyl)-2-aminopropane (2,5-DMA) derivatives, most of which are hallucinogenic in man, and several related agents were determined for rat cortical serotonin (5-HT) binding sites. Competition assays were performed in which these agents were competed for the 5-HT2 binding of [3H]ketanserin, or the 5-HT1 binding of [3H]LSD (in the presence of ketanserin). The R(-)-isomers of DOI, DOM and DON (i.e. the 4-iodo, -methyl and -nitro derivatives of 2,5-DMA) were found to be more potent than their racemates and demonstrated selectivity for 5-HT2 sites. These same agents in competing for [3H]ketanserin binding resulted in Hill coefficients significantly less than unity; computer-assisted analysis indicated a two-state model better fit the data. In the presence of 10(-4) M Gpp(NH)p the competition curve for R(-)-DOI produced a Hill coefficient close to unity. These results are consistent with the hypothesis that certain derivatives of 2,5-DMA, in particular R(-)-DOI, may be potent and selective agonists at 5-HT2 binding sites, sites that may constitute a serotonin receptor that is regulated by a guanine nucleotide regulatory protein. Conversely, the interactions of these agents at 5-HT1 sites was with a lower affinity and a lack of stereoselectivity. Although DOI and DOM are amongst the most potent of these agents as hallucinogens, it is still too premature to draw any conclusions regarding a possible relationship between 5-HT binding and hallucinogenic potency.