Guanylin/GCAP-I and uroguanylin/GCAP-II are two structurally related peptides which play an important role in the regulation of water/electrolyte balance within the gut. In order to enable the investigation and comparison of both peptide hormones at the genomic level, we decided to clone the corresponding genes. The human gene for guanylin/GCAP-I and its 5'-flanking region have been described recently. Here, we report the three exon/two intron structure of the human uroguanylin/GCAP-II gene and its localization on chromosome 1 p35-34, as determined by radiation hybrid mapping. Together with data obtained for the guanylin/GCAP-I gene we show that these genes are localized in the same chromosomal area with other guanlyl cyclase-activating peptides like ANP etc. Northern hybridization revealed that the expression of the uroguanylin/GCAP-II gene is highest in the intestinal mucosa, especially in the ileum and colon. By means of polymerase chain reaction (PCR), an expression was also observed in the stomach where no guanylin/GCAP-I expression is detectable. Using immunohistochemical methods, uroguanylin/GCAP-II immunoreactive material was distinctly localized in D-type gastric and intestinal endocrine cells. Although the comparable data on the genomic organisation of both peptide hormones verify their high degree of relationship, this finding indicates a special task of uroguanylin/GCAP-II within the stomach, such as regulatory functions in gastric secretion. The redundant expression of the GCAP/GC-C system in the small and large intestine, however, is as yet unclear.