Using the polymerase chain reaction with degenerate primers to identify novel G-protein-coupled receptors of the rat alveolar Type II cell, we identified sequences expressed by the Type II cell identical to the sequence of the rat brain cannabinoid receptor (CB₁). The use of Northern blot analysis to examine expression of CB₁ mRNA in rat tissues revealed differences between the brain and lung. While rat brain expressed a 6.0 kb mRNA as previously described, rat lung expressed mRNA of 4.5 and 6.0 kb. Isolated lung alveolar Type II cells also expressed mRNA of 4.5 and 6.0 kb as determined by Northern analysis. However, only freshly isolated Type II cells contained cannabinoid receptor mRNA. Reverse transcriptase-polymerase chain reaction (RT-PCR) failed to detect CB₁ mRNA in Type II cells maintained in culture for 1 or 2 days. We next determined developmental changes in lung CB₁ mRNA expression using semi-quantitative RT-PCR. CB₁ expression was detected as early as gestational day 16 in rat lung and mRNA levels increased to fetal day 20 before birth, before declining to adult levels. Fetal rat lung explants were utilized to further examine the ontogeny and hormonal effects on CB₁ mRNA expression. Hydrocortisone induced a dose-dependent expression in 15-day and 18-day explants, similar to previous results for surfactant-associated proteins. Our results demonstrate expression of CB₁ mRNA in rat alveolar Type II cells and rat lung. This expression is ontogenically and hormonally regulated, with maximal expression noted just prior to birth in rat lung. Since CB₁ mRNA is only expressed in freshly isolated Type II cells, CB₁ may be useful as a Type II cell marker.