1. We have used previously characterized clones of the human neuroblastoma cell line, SH-SY5Y. constitutively expressing either the human 5-HT2A or 5-HT2C receptor to compare their desensitization profiles after exposure to 5-HT. 2. 5-HT stimulated [3H]-inositol phosphate ([3H]-IPx) production at both the 5-HT2C (pEC50=8.03+/-0.15) and 5-HT2A receptors (pEC50=7.15+/-0.08), with maximal responses occurring after exposure to 1 microM and 10 microM 5-HT, respectively. 3. Exposure of cells to maximally effective concentrations of 5-HT caused time- and concentration-dependent desensitization of [3H]-IPx formation. The 5-HT2A response desensitized slower (t1/2 = 110 min) and with lower sensitivity than that of the 5-HT2C receptor (t1/2 = 12.5 min). In each case, desensitization was blocked by co-administration of a specific receptor antagonist. Following exposure to 10 microM 5-HT for 2 h, both receptors exhibited extensive desensitization, with subsequent responses to 5-HT reduced by more than 80%. 4. 5-HT stimulated Ins(1,4,5)P3 production with a potency similar to that for [3H]-IPx production at each receptor. In both cases Ins(1,4,5)P3 levels peaked rapidly then returned to basal level within a short time. This peak consistently occurred earlier for the 5-HT2C receptor (5 s) than for the 5-HT2A receptor (20 s). 5. Prior exposure of SH-SY5Y/5-HT2C cells to 5-HT (1 microM/15 min) caused a significant decrease in the 5-HT-stimulated peak in Ins(1,4,5)P3 levels whereas no such change occurred in SH-SY5Y/5-HT2A cells following exposure to 10 microM 5-HT for 15 min. 6 These results indicate that the human 5-HT2A and 5-HT2C receptors both exhibit desensitization at the level of inositol phosphate formation when expressed in the same cellular environment, with the 5-HT2C receptor being more sensitive to 5-HT-mediated desensitization than the 5-HT2A receptor.