The expression and activities of corticotrophin-releasing factor (CRF), urocortin (UCN), the CRF-binding protein (CRF-BP) and CRF receptors in rat brain have been well documented; however, information regarding their peripheral distributions remains incomplete. Given the multiple immunomodulatory effects of peripherally administered CRF and UCN and the high levels of CRF receptor type 2 (CRF-R2) mRNA and protein expressed in the heart, the lymphoid organs and heart have become targets for some of the latest CRF-related research. Here we demonstrate the presence of UCN mRNA in both the rat spleen and human Jurkat T-lymphoma cells using 3'-RACE (rapid amplication of cDNA ends) PCR. Following on from these initial results, we used semi-quantitative RT-PCR to carry out a comprehensive study assessing the relative amounts of CRF, UCN, CRF-R1, CRF-R2 and CRF-BP mRNAs in the brain, thymus, spleen and heart of normal, untreated rats. The rank orders of mRNA abundance in each of the tissue types were as follows: for CRF, brain>>thymus=spleen=heart; for UCN, heart>/=brain>thymus>spleen; for CRFR1, brain>>thymus>spleen (absent in heart); for CRF-R2, brain=heart>thymus>spleen; and CRF-BP was only detectable in the brain. We have provided evidence for the existence of CRF, UCN, CRF-R1 and CRF-R2 expression in resting immune cells, with UCN expression being particularly predominant in the rat thymus and human Jurkat cells. Additionally, the high levels of UCN mRNA detected in heart corresponded to the high expression of CRF-R2 mRNA, suggesting an important role for UCN/CRF-R2 coupling in this tissue.