We describe ribonuclease protection assays for dopamine D1, D2, and D3 receptors, and regional quantitation of mRNA levels for these receptors in rat brain. Both D1 and D2 mRNA levels were highest in caudate putamen, where they were found in approximately equal levels. Of the brain regions examined, D3 mRNA was most abundant in hippocampus, hypothalamus, and nucleus accumbens. Levels of D3 mRNA were significantly lower than values for D1 and D2 mRNA in all brain regions studied. Variability was observed between animals for expression of both D1 and D2 mRNA in caudate putamen, with a significant correlation between D1 and D2 mRNA levels in neostriatum (r = 0.942, P < 0.001). This suggests a functional interaction between D1 and D2 receptor mRNA levels in this brain region. Our results are generally consistent with regional distributions previously reported using other methods. These results suggest that DA D2 receptors function both as an auto and as a postsynaptic receptor, while D1 receptors are restricted to a postsynaptic function. Our results demonstrate the utility of this method in studying possible relationships between individual animal variation in regional mRNA expression and behavioral response to pharmacological and other experimental treatments.