The patterns of dipeptidyl peptidase (DPP) IV activity and protein amount in different rat organs during development were compared. In order to elucidate the molecular basis for these patterns, total RNA was isolated from lung and kidney at different stages of development and analysed by Northern-blot hybridization using an oligonucleotide derived from the DPP IV cDNA sequence. This oligonucleotide hybridized to two distinct mRNAs of approx. 3.2 and 4.8 kb respectively. During kidney development, the pattern for DPP IV mRNA paralleled that of DPP IV activity and protein amount, suggesting that, in kidney, the expression of DPP IV is primarily controlled at the transcriptional level. In contrast, the magnitude of DPP IV activity during lung development compared with that of DPP IV mRNA in lung suggests that post-transcriptional mechanisms are involved in regulating the expression of DPP IV in lung. Organ-specific regulation of DPP IV expression may provide a useful model for further comparative studies of transcriptional and post-transcriptional mechanisms of DPP IV expression within the same organism.