Previous studies demonstrated that, compared with long-chain TAG (LCT), dietary medium-chain TAG (MCT) could improve glucose tolerance in rats and humans. It has been well established that adiponectin acts to increase insulin sensitivity. The effects of dietary MCT on adiponectin serum concentration and mRNA levels in adipose tissue were studied in rats. Male Sprague-Dawley rats were fed a diet containing 20% MCT or LCT for 8 wk. After 6 wk of dietary treatment, an oral glucose tolerance test was performed. Rats fed the MCT diet had less body fat accumulation than those fed the LCT diet (P < 0.01). The cell diameter of the perirenal adipose tissue, one of the abdominal adipose tissues, was smaller (P < 0.01) in the MCT diet group. The serum adiponectin concentration was higher (P < 0.01) in the MCT diet group than in the LCT diet group. The adiponectin content in the perirenal adipose tissue was higher (P < 0.01) in the MCT diet group. The MCT-fed group had a higher adiponectin mRNA level in their perirenal adipose tissue (P < 0.05). The increase of the plasma glucose concentration after glucose administration (area under the curve) was smaller (P < 0.01) in the MCT diet group than in the LCT diet group. These findings suggest that dietary MCT, compared with LCT, results in a higher serum adiponectin level with transcriptional activation of the adiponectin gene in rats. We speculate that improved glucose tolerance in rats fed an MCT diet may be, at least in part, ascribed to this higher serum adiponectin level.