We investigated the usefulness of reverse transcriptase-polymerase chain reaction (RT-PCR) to quantify glucose transporter 1 (GLUT1) mRNA in cerebral microvessels. The technique was validated using an in vitro-transcribed RNA fragment (riboprobe) of partial (3^{\prime}) noncoding sequence of rat brain GLUT 1 gene. A known amount of the riboprobe was reverse-transcribed to cDNA (target DNA). PCR primers were made to amplify a 292-bp fragment of the target DNA. The (5^{\prime}) primer was end labeled with ({ }^{32} P). An oligonucleotide of (100 \mathrm{bp}) containing the same sequences as the first 30 and the last 70 bases of the 292 -bp fragment of the target DNA was synthesized and used as competitive DNA. The target DNA was coamplified with increasing amounts of competitive DNA using the same two primers. The ratio of radioactivity between amplified products of the target DNA ( 292 -bp fragment) and the competitive DNA ( (100-) bp fragment) was determined quantitatively after separation by gel electrophoresis and radioactivity counting. This method gave an accurate estimation of the amount of the riboprobe in the reaction and a 2 - to 5 -fold change in the amounts could be detected. By this method, the mean amount of GLUT1 mRNA from purified rat brain microvessels was estimated to be (1.5 \pm 0.1 \times 10^{-8} \mathbf{n g} / \mathrm{ng}) total RNA. This value was about 10 -fold higher than that in rat cell line PC12.