Two dopamine 1-like receptors linked to the stimulation of adenylyl cyclase have been cloned in mammals, dopamine 1A and 1B (D1A and D1B). D1 receptors have been localized to various nephron segments using radioligand binding studies. These methods, however, fail to distinguish between the D1A and D1B receptor subtypes. We, therefore, studied D1 receptor subtype expression in different nephron segments using reverse transcriptase polymerase chain reaction (RT-PCR), in-situ RT-PCR, western blotting, and quantitative-competitive (QC) RT-PCR using cDNA mutants for the D1A and D1B receptor. Proximal convoluted tubule (PCT) and cortical collecting duct (CCD) were obtained by microdissection. Proximal and distal tubules and medullary thick ascending limb of Henle (mTAL) were also obtained by sieving and differential centrifugation. RT-PCR showed both D1A and D1B receptor mRNA in microdissected PCT and CCD; in mTAL, D1B but not D1A mRNA was detected. QC RT-PCR showed that D1A mRNA in proximal tubules was 110 fg/μg total RNA while D1A mRNA in distal tubules was 1 fg/μg total RNA. D1B mRNA level in proximal tubules was 50 fg/μg total RNA and 1fg/μg total RNA in distal tubules. Western blotting showed that D1A and D1B receptor protein expression was greater in PCT than in CCD; in mTAL, D1B but not D1A receptor was present. These studies showed quantitatively for the first time that D1A receptor mRNA was present in proximal tubules in greater abundance than D1B receptor mRNA. D1A and D1B receptor mRNA were expressed in approximately equal abundance in distal tubules. Additionally, D1A and D1B receptor protein expression was greater in PCT than in CCD. In the mTAL, D1B but not D1A receptor mRNA and protein was present. The presence of D1B but not D1A receptor in the mTAL and the differential expression of the D1 receptor subtypes may explain the differences in D1 signal transduction in these nephron segments.