In this study we report the isolation and characterization of three overlapping cDNA clones for the type I/3 isozyme of cGMP-dependent protein kinase (cGK) from human placenta libraries. The composite sequence was 3740 nucleotides long and contained 58 nucleotides from the S-noncoding region, an open reading frame of 2061 bases including the stop codon, and a 3'noncoding region of 1621 nucleotides. The predicted full-length human type IB cGK protein contained 686 amino acids including the imtiator me~o~ne, and had an estimated molecular mass of 77 803 Da. On comparison to the published amino acid sequence of bovine lung Ia, human placenta I/I cGK differed by only two amino acids in the carboxyl-terminal region (amino acids 105686). In contrast, the amino-terminal region of the two proteins was markedly different (only 36% similarity), and human I/I cGK was 16 amino acids longer. In a specific region in the aminoterminus (amino acids 6375) 12 out of 13 amino acids of the human I/f cGK were identical to the partial amino acid sequence recently published for a new I/l isoform of cGK from bovine aorta. Northern blot analysis demonstrated a human I/l cGK mRNA, 7 kb in size, in human uterus and weakly in placenta. An mRNA of 7 kb was also observed in rat cerebellum, cerebrum, lung, kidney, and adrenal, whereas an mRNA doublet of 7.5 and 6.5 kb were observed in rat heart. Comparison of Northern and Western blot analyses demon~rat~ that the mRNA and protein for cerebellar cGK increased during the development of rats from 5 to 30 days old, whereas the 6.5 kb mRNA in rat heart declined.