Zinc-alpha(2)-glycoprotein (ZAG), a lipid-mobilizing factor, is a novel adipokine which may act locally to influence adipose tissue metabolism. This study examined the ontogeny of ZAG expression in adipose tissue during postnatal development. White (subcutaneous, gonadal, and perirenal) and brown (interscapular) fat was collected from rats aged 1-32 days. ZAG mRNA was detected from day 1 in subcutaneous fat, which appears the key site of synthesis postnatally. ZAG was detected in perirenal (day 5) and in gonadal (day 7) fat when the tissue was sufficient for analysis. ZAG mRNA and protein levels fell significantly at weaning (day 21) and thereafter. ZAG was also detected in brown fat at day 1 but it fell significantly afterwards. The downregulated ZAG expression in white fat depots at weaning, when fat mass is expanded substantially, suggests that ZAG might be involved in the postnatal development of adipose tissue mass.