Forty-eight male rats were fed a nutritionally complete diet containing 30% of dietary energy as fat. For 24 animals (control) the fat source was corn oil, for the remaining 24 rats (experimental) the fat source was a triundecanoin-corn oil mixture (7:3, wt/wt). After 6 wk, groups of control and experimental rats were killed after 0, 24, and 48 h of fasting. In the experimental group, adipose tissue fatty acids contained, on average, 280 mmol undecanoate/mol fatty acid. In the control group, no odd-numbered fatty acids were present. During fasting, the experimental groups had higher plasma glucose and alanine levels, higher plasma insulin-to-glucagon ration, and lower liver phosphenol pyruvate caboxykinase. The results suggest that the terminal propionate residues generated when odd carbon fatty acids are oxidized become gluconeogenic precursors and cause a reduced need for gluconeogenesis from protein.