We have developed a neurotensin analog, L-[3,1'-naphthylalanine11]NT(8-13), NT34, that can distinguish between rat and human neurotensin receptors, and exhibits more than a 100-fold difference in binding affinities and a 60-fold difference in functional coupling to phosphatidylinositol turnover. Using cells transfected with different numbers of the appropriate receptors, we measured the changes in phosphatidylinositol production, and then evaluated the efficiency of receptor-effector coupling based on Furchgott's design. The binding of NT34 at both rat and human neurotensin receptors stably expressed in CHO-K1 cells was to two sites, while the binding of NT was to one site. At the rat receptor the equilibrium dissociation constant (Kd) for NT34 at the high-affinity site was 0.058 nM, while that at the low-affinity site was 3.1 nM. For the human receptor at the high-affinity site, the Kd for NT34 was 18 nM, while that at the low-affinity site was 180 nM. For both species the percentage of receptors representing the high-affinity site was approximately 60-70% with 30-40% at the low-affinity site. We derived agonist dissociation constants (Ka) for NT and NT34, which suggest that for NT34, the low-affinity site is functionally coupled to phosphatidylinositol turnover. Finally, we compared the relative efficacies of both compounds and found that NT34 was about 2-fold and 4-fold more efficacious than NT in stimulating phosphatidylinositol turnover in rat and human NT receptors, respectively.