The mu opioid receptor (MOR-1) mRNA was quantified in rat CNS by a sensitive solution hybridization (SH) technique, employing a 32P-labeled riboprobe derived from the coding region of MOR-1 cDNA. In a Northern blot analysis this riboprobe hybridized to a 14 kb form of rat MOR-1 mRNA. The linear range of SH assay extends from 1 to 250 pg of MOR-1 sense transcript (equivalent to 9.3-2325 pg of MOR-1 mRNA). A microdissection technique for reproducible sampling of selected CNS regions, followed by the SH assay, allowed for a quantitative study of MOR-1 mRNA distribution. The highest levels of MOR-1 mRNA were present in medial thalamus (17.8 +/- 0.3 pg/micrograms RNA), and the lowest in the cerebellum (0.4 +/- 0.1 pg/microgram RNA). Hypothalamus, dorsal spinal horn, nucleus raphe, periaqueductal gray, and sensorimotor cortex contained intermediate levels. This distribution closely parallels the pattern of mu receptor binding, suggesting that both the mRNA and the receptor protein are colocalized within most of the regions studied.