A series of arylpiperazines was examined for structure-function relationships at the human platelet serotonin (5-HT) receptor. Amplification of ADP-induced aggregation was used to measure 5-HT receptor activation. The platelet serotonergic agonists 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), 5-HT and 5-methoxytryptamine (5-MeOT) and the antagonist ketanserin were used for comparison of potency and amplitude of response. All arylpiperazines, including the parent compound phenylpiperazine (PP) showed antagonist activity. The monosubstituted phenylpiperazines acted only as antagonists, and electron-withdrawing substituents markedly enhanced activity. Modification of PP by addition of another phenyl ring or benz-fusion also enhanced antagonist activity. Benz-fusion at the b face of PP (1-NP) yielded greater antagonist potency than benz-fusion at the c face (2-NP). The latter modification, however, also conferred a variable agonist activity with a very weak response. In contrast, the heteroaromatic piperazines consistently demonstrated concentration dependent mixed antagonist-agonist activity. These compounds were weak agonists compared with 5-HT, 5-MeOT and DOI, although the amplitude of the quipazine response was similar to DOI. This study demonstrates that the arylpiperazines, which are variably selective for the multiple brain 5-HT receptors, are all antagonists on the platelet 5-HT receptor. The antagonist activity is markedly increased by ring monosubstitution or aryl modification. Compared with the monosubstituted analogues, antagonist activity is decreased by heteroaromatic modification or by the addition of an N-aminophenethyl group to the 4-position nitrogen. Weak agonist activity can be conferred by heteroaromatic modification.