The adipose tissue-derived factor resistin has been suggested to induce insulin resistance in obesity and inhibit adipocyte differentiation. Lactation is associated with major metabolic adaptations. which cause a profound loss of adipose tissue to support milk production by the mammary gland. This study has examined the expression of the resistin and resistin-like molecule-alpha (RELM-alpha) genes in white adipose tissue of lactating mice, Lactation induced significant increases in food intake and body weight, whereas body fat was substantially decreased at peak lactation. Resistin and RELM-alpha mRNAs were both detectable in gonadal. subcutaneous, and mammary gland fat; mRNA level was highest in gonadal fat and lowest in mammary tissue. There was no difference in resistin mRNA level in gonadal fat of lactating mice compared with controls. However. RELM-alpha mRNA fell by approximately 40% in early lactation and there was a parallel fall in the leptin mRNA level proportional to the loss of fat mass. These results indicate that the substantial reduction of adiposity in lactation does not lead to any changes in resistin gene expression: however. the Call in RELM-alpha mRNA might indicate a role for RELM-alpha in the metabolic adaptations of lactation, (C) 2002 Elsevier Science (USA). All rights reserved.