The expression of 3 neuropeptide genes, vasopressin (AVP), oxytocin (OT) and somatostatin (SOM), was studied in the developing rat hypothalamus using Northern blot analysis combined with densitometric scanning. A unique profile of developmental expression was established for each of the 3 genes. SOM mRNA is detectable at embryonic day 14 and reaches 40% of the adult levels by embryonic day 18. By contrast, accumulation of AVP and OT mRNA is mainly a postnatal event. AVP mRNA, although detectable in the late embryo, rises gradually after birth and attains 40% of adult levels after the second postnatal week. Maturation of OT gene expression occurs even later and parallels AVP gene expression with a lag time of one week. Observed increases in mRNA levels are due to an upregulation of gene expression since they occur essentially following cessation of neuronal cell proliferation. The rise in AVP and OT mRNA accumulation coincides with the establishment of synaptic input to AVP and OT neurons. Expression of the SOM gene, by contrast, occurs prior to neuronal cell differentiation and points to a possible function of SOM in the embryonic brain.