1. Factors influencing agonist affinity and relative efficacy have been studied for the 5-HT(1A) serotonin receptor using membranes of CHO cells expressing the human form of the receptor and a series of R-and S-2-(dipropylamino)tetralins (nonhydroxylated and monohydroxylated (5-OH, 6-OH, 7-OH, 8-OH) species). 2. Ligand binding studies were used to determine dissociation constants for agonist binding to the 5-HT(1A) receptor: (a) K(i) values for agonists were determined in competition versus the binding of the agonist [(3)H]-8-OH DPAT. Competition data were all fitted best by a one-binding site model. (b) K(i) values for agonists were also determined in competition versus the binding of the antagonist [(3)H]-NAD-199. Competition data were all fitted best by a two-binding site model, and agonist affinities for the higher (K(h)) and lower affinity (K(l)) sites were determined. 3. The ability of the agonists to activate the 5-HT(1A) receptor was determined using stimulation of [(35)S]-GTPgammaS binding. Maximal effects of agonists (E(max)) and their potencies (EC(50)) were determined from concentration/response curves for stimulation of [(35)S]-GTPgammaS binding. 4. K(l)/K(h) determined from ligand binding assays correlated with the relative efficacy (relative E(max)) of agonists determined in [(35)S]-GTPgammaS binding assays. There was also a correlation between K(l)/K(h) and K(l)/EC(50) for agonists determined from ligand binding and [(35)S]-GTPgammaS binding assays. 5. Simulations of agonist binding and effect data were performed using the Ternary Complex Model in order to assess the use of K(l)/K(h) for predicting the relative efficacy of agonists.