1. [75-Se]selenomethionine was administered to four rabbits and after 4 d their kidneys were removed and homogenized. The long-term fate in rats of an oral dose of this kidney homogenate (RK-75-Se) was compared with that of an oral dose of ]75-Se]selenomethionine mixed with unlabelled rabbit kidney homogenate. 2. Urinary adn faecal radioactivities were measured during the 1st week and whole-body radioactivity was determined for 10 weeks. Rats were killed at weekly intervals for 4 weeks for analysis of tissue distribution of 75-Se. 3. Intestinal absorption of RK-75-Se was 87%; that of [75-Se]selenomethionine was 91%. Urinary excretion of absorbed RK-75-Se was 13-3% and that of [75-Se]selenomethionine was 7-6%, in the 1st week. 4. Whole-body retention of 75-Se was greater for [75-Se]selenomethionine than for RK-75-Se but after the 1st week decreased at a similar rate in both groups. Tissue distribution of retained 75-Se was also similar in both groups. 5. The initial utilization of 75-Se in rabbit kidney is different from that of [75-Se]selenomethionine. However, after the 1st week 75-Se from these sources appears to be metabolized similarly, suggesting that Se from both is ultimately incorporated into the same metabolic pool.