The effects of a diet rich in alpha-linolenic acid vs. one rich in oleic acid on the oxidation of uniformly labeled C-13-alpha-linolenic acid and its conversion into longer-chain polyunsaturates (LCP) were investigated in vivo in healthy human subjects. Volunteers received a diet rich in oleic acid (n = 5) or a diet rich in alpha-linolenic acid (n = 7; 8.3 g/d) for 6 wk before and during the study. After 6 wk, subjects were given 45 mg of C-13-alpha-linolenic acid dissolved in olive oil. Blood samples were collected at t = 0, 5, 11, 24, 96, and 336 h. Breath was sampled and CO2 production was measured each hour for the first 12 h. The mean (+/- SEM) maximal absolute amount of C-13-eicosapentaenoic acid (EPA) in plasma total lipids was 0.04 +/- 0.01 mg in the alpha-linolenic acid group, which was significantly lower (P = 0.01) than the amount of 0.12 +/- 0.03 mg C-EPA in the oleic acid group. Amounts of C-13-docosapentaenoic acid (DPA) and C-13-docosahexaenoic acid (DHA) tended to be lower as well. The mean proportion of labeled a-linolenic acid (ALA) recovered as (CO2)-C-13 in breath after 12 h was 20.4% in the ALA and 15.7% in the oleic acid group, which was not significantly different (P = 0.12). The cumulative recovery of C-13 from C-13-ALA in breath during the first 12 h was negatively correlated with the maximal amounts of plasma C-13-EPA (r = -0.58, P = 0.047) and C-13-DPA (r = -0.63, P = 0.027), but not of C-13-DHA (r = -0.49, P = 0.108). In conclusion, conversion of C-13-ALA into its LCP may be decreased on diets rich in ALA, while oxidation of C-13-ALA is negatively correlated with its conversion into LCP. In a few pilot samples, low C-13 enrichments of n-3 LCP were observed in a diet rich in EPA/DHA as compared to oleic acid.