The relationship of leptin gene expression to adipocyte volume was investigated in lean 10-wk-old male C57BL/6J mice. mRNA levels for leptin, insulin receptor, glucocorticoid receptor, and tumor necrosis factor (TNF)-alpha in inguinal, epididymal, and retroperitoneal adipose tissues were quantified and related to adipocyte volume. Leptin mRNA levels were highly correlated with adipocyte volume within each fat depot. Multiple regression analysis of pooled data from the three depots showed that leptin mRNA levels were strongly correlated with adipocyte volumes (beta = 0.84, P < 0.001) and, to a smaller degree, with glucocorticoid receptor mRNA levels (beta = 0.36, P < 0.001). Depot of origin had no effect (P > 0.9). Rates of leptin secretion in vitro were strongly correlated with leptin mRNA levels (r = 0.89, P < 0.001). mRNA levels for TNF-alpha, insulin receptor, and glucocorticoid receptor showed no significant correlation with adipocyte volume. These results demonstrate that depot-specific differences in leptin gene expression are mainly related to the volumes of the constituent adipocytes. The strong correlation between leptin gene expression and adipocyte volume supports leptin's physiological role as a humoral signal of fat mass.