The human genes for prepro-arginine-vasopressin-neurophysin II (prepro-AVP-NPII) and prepro-oxytocin-neurophysin I (prepro-OT-NPI) were cloned from a human genomic library and the nucleotide sequence of both genes was determined. The two genes are similar in their intron-exon structure, linked together with 12 kilobases intervening, and transcribed from opposite DNA strands. A human small cell lung cancer cell line, H378, produces significant quantities of pre-pro-AVP-NPII mRNA using a transcription unit predicted from the genomic DNA sequence. Despite the proximity of the actively transcribed prepro-AVP-NPII gene, transcription of prepro-OT-NPI is not detected in this cell line.