The rat liver expresses two low molecular weight kininogens (T-KG and K-KG). Although they share 90% of the nucleotide sequence in their 5' flanking regions, T- and K-KG genes are differentially regulated. The T-KG gene is inducible, and its protein is a potent thiol-protease inhibitor. In contrast, K-KG gene is expressed constitutively and encodes the precursor of the vasoactive nonapeptide bradykinin. To further elucidate the differential regulation of T- and K-KG genes, we examined their developmental expression in the Sprague-Dawley rat. Northern blots of total liver RNA were probed with oligonucleotides complementary to T and K-KG mRNA under high-stringency conditions. A single T-KG mRNA (1.8 kb) and two K-KG mRNA species (1.6 and 2.3 kb) were consistently detected at all ages studied. Steady state T-KG mRNA levels increased 3.5-fold at birth and remained high during the 1st week of postnatal life only to decline thereafter. T-KG immunoreactivity in the liver and plasma determined by Western blot analysis paralleled T-KG mRNA expression. In marked contrast, K-KG mRNA expression was not altered during the transition from fetal to neonatal life, nor was it affected by postnatal maturation. The results demonstrate that the fetal rat liver synthesizes kininogens and that T- and K-KG genes are differentially regulated during development. Up-regulation of T-KG synthesis after birth may serve a protective function in the newborn via its antiprotease activity.(ABSTRACT TRUNCATED AT 250 WORDS)