1. The binding characteristics of tritium labeled 8-hydroxy-dipropyl-aminotetralin, or [3H]8-OH-DPAT, to the serotonin1A (5-HT1A) receptor in the stably transfected HeLa cell clone HA6 and in human cortical tissue were examined and compared. 2. A series of kinetic studies of [3H]8-OH-DPAT binding to the transfected HA6 cell line demonstrated two components in both the association and the dissociation reactions. 3. In saturation experiments, at least two affinity states were unequivocally detected in the HA6 cell line and the human cortical tissue. Using isotopic dilutions, the binding isotherms were best fitted to a two-site model, and similar affinity values were obtained in both systems (KH approximately 1.1 nM and KL approximately 12-223 nM). 4. Most of the drugs used in competitions inhibited [3H]8-OH-DPAT binding, following a two-site model, and maintained their rank order of binding potency in both systems; that is, 5-HT > or = 8-OH-DPAT > buspirone > pindolol. Inconsistencies, however, were found for the antagonists NAN-190 and pindolol; only one inhibition constant was determined for HA6 cells, but two affinities were detected with cortical tissue. 5. The results indicate that, although data from binding studies using the cell expression system reflect, to a certain extent, those obtained with the cortical tissue, some discrepancies remained. 6. Finally, and in contrast with what is observed with the 5-HT1A receptor expressed in the HA6 cell line, it is possible that different receptors, or subtypes of one receptor, or even uptake sites normally expressed in cortical tissue, could interact with [3H]8-OH-DPAT or the competing drugs or both, thus leading to the observation of additional binding sites.