The contractile response of the isolated rabbit aorta elicited by 5-hydroxytryptamine (5-HT) and five partial agonists acting on the 5-HT2 receptor were separated into a phasic and a tonic response by altering the [Ca++] in the buffer. A kinetic analysis of the two responses yields parameters that provide a mechanistic insight into the different nature of these responses. The kinetic parameters of the phasic contraction indicate that the onset of this response depends on the access of the drug to the receptor and that its decay is independent of the nature and the concentration of the agonist. The observed rate constant of the onset of the tonic response, kobs, is saturable with increasing drug concentration, suggesting that the rate determining step is the activation of an effector by the preformed drug-receptor complex. These kinetic characteristics of the 5-HT2-mediated response are similar to those observed previously by us for the alpha-1 adrenergic receptor-mediated response in the rabbit aorta, suggesting that these receptors activate similar mechanisms related to the mobilization of Ca++. Furthermore, it is shown that the maximal values of kobs for the 5-HT2 agonists follow the rank order of maximal amplitudes of the phasic responses and the maximal steady-state levels of the tonic response. It is suggested that the maximal value of kobs may serve as a kinetic measure of drug efficacy.