The expression of plasma prekallikrein (PPK) mRNA has been investigated applying reverse transcription, followed by polymerase chain reaction, of mRNA (RT-PCR) in various human and rat tissues. PPK gene transcripts were detected in liver and kidney in both species and, in addition, in human adrenal gland and placenta. No PPK mRNA was identified in rat adrenal gland, heart, aorta, lung, brain cortex and medulla, hypothalamus, and uterus. These results show that PPK gene expression is not restricted to the liver.