The pressor responses to six selective 5-HT receptor agonists, quipazine, TMFPP, Org 10155, Ru 24969, 5-methoxytryptamine and 5-methoxy-N,N-dimethyltryptamine, were quantified and tested for calcium dependency following i.v. administration to pithed normotensive rats. The pressor responses induced by 5-methoxytryptamine and 5-methoxy-N,N-dimethyltryptamine were not influenced by calcium entry blockade by means of nifedipine. The piperazine derivatives quipazine, TMFPP and Org 10155 were full agonists, but the pressor responses to these agonists were sensitive to calcium entry blockade. 5-Methoxy-N,N-dimethyltryptamine and Ru 24969 were found to be partial agonists. It is submitted as a hypothesis that within the present series of 5-HT receptor agonists, a restricted side chain flexibility will enhance the dependency of their pressor responses to extracellular calcium.