Agonist regulation of 5-hydroxytryptamine2 (5-HT2) receptors was studied in calf aortic smooth muscle cultures incubated in a quiescent, defined synthetic medium that does not stimulate cell proliferation, but that provides cells with supplements that maintain cell viability. In these cells, 5-hydroxytryptamine (5-HT)-induced [3H]inositol phosphates accumulation showed the characteristics of a 5-HT2 receptor coupled transducing system according to the inhibition of the response by 5-HT2 antagonists at nanomolar concentrations. The 5-HT2 receptor coupled response became rapidly desensitized during continued incubation with 5-HT and 1-(2,5-dimethoxy-4-methylphenyl)-2- aminopropane (DOM); nearly full desensitization was obtained in two hours with 10 microM 5-HT and DOM pretreatment. The recovery of the response had a half-live of 5 hours after 2 hours pretreatment and of 9.5 to 12.5 hours after 24 to 96 hours agonist pretreatment. The DOM-induced desensitization of the 5-HT2 receptor coupled response was fully blocked by 0.1 microM cinanserin. Cinanserin alone did not induce desensitization or up-regulation of the 5-HT2 receptor coupled response at 0.1 microM. It may be that the down-regulation of central 5-HT2 receptors by antagonists in vivo is a heterologous process due to mediators which are triggered by 5-HT2 antagonistic action.