To determine the extent to which suckling animals differ from adults in their capacity to absorb fat, we compared the rate of absorption of orally administered [14C]triolein in 11- to 12-day-old suckling rats with that of 10-wk-old adults by three distinct methods. In the first, the rate of [14C]triolein disappearance was determined by quantitating substrate remaining in the gastrointestinal tract after oral administration. In the second, 14CO2 expiration in breath was measured continuously for 6 h after an identical feeding. In the third, intestinal triglyceride output was estimated by the lipoprotein lipase inhibitor, Triton WR-1339. Triolein disappearance, 14CO2 excretion, and intestinal triglyceride output were two- to threefold higher in suckling rats compared with adults (P less than 0.01, P less than 0.0001, and P less than 0.01, respectively). There was also a highly significant linear relationship between 14CO2 excretion and both triolein disappearance and intestinal triglyceride output for both age groups (P less than 0.0001 and P less than 0.003, respectively). These data show that consistent with its high dietary intake, the suckling rat can absorb triolein at rates significantly higher than the adult.