Levels of endogenous neurotensin (NT-IR) in the LS x SS RI strains differed by 3.0-, 4.7-, 5.4-, and 6.9-fold in the ventral midbrain (VMB), hypothalamus (HY), nucleus accumbens (NA), and caudate putamen (CP), respectively. Frequency distributions and estimates of the number of genes indicate that differences in NT-IR are polygenically influenced. The NT-IR levels in NA and CP were significantly correlated, but levels in the VMB did not correlate with those in the NA or CP. Specific binding to either low (NTL)- or high (NTH)-affinity receptors as measured in the absence or presence of levocabastine differed significantly in brain regions from among LS X SS mouse strains. Results indicate a polygenic influence mediating the differences in receptor densities and suggest differences in genetic regulation of NTL and NTH receptors.