Attempts to quantify renal prothrombin (PT) have failed due to interference of its hepatic counterpart. In order to gauge PT synthesized by the kidney, the expression of PT mRNA was compared in renal and hepatic tissues of rats. PT mRNAs were quantified, using quantitative competitive reverse transcription-polymerase chain reaction (RTPCR), from RNA extracts of kidneys and livers of ten hooded Wistar rats. To control variations in sample preparation, the amount of PT transcript in each sample was normalized to the amount of its beta-actin transcript. The median ratio of PT to beta-actin transcript of 0.0011 (with a mean +/- standard deviation (SD) of 0.0010 +/-0.0002, range 0.0007-0.0014) in the renal tissues was significantly (p less than or equal to 0.001) lower than its corresponding value of 0.3669 (with a mean +/- SD of 0.3729 +/- 0.0716, range 0.27180.4675) in the hepatic tissues. Thus, expression of PT mRNA in the rat kidney was almost 300-fold less as compared with its expression in hepatic tissue.