A fast and reliable hplc method for the quantitative analysis of total fecapentaene-12 (FP-12 and its precursors) and total fecapentaene-14 (FP-14 and its precursors) in human feces is described. The analysis is based on the rapid chemical conversion of fecapentaenes and their precursors to more stable methoxytetraenols and the use of synthetic, not naturally occurring, fecapentaene-13 (FP-13) as an internal standard. The synthesis and physical properties of this internal standard are described. The convenience and reproducibility of the method were illustrated by applying the procedure to stool samples obtained from twelve individuals on 3 consecutive days. Levels of total FP's were in the range of 0.1-25.4 micrograms for total FP-12 and 0.1-8.5 micrograms for total FP-14 per g of wet feces. Appreciable fluctuations were observed between levels in samples from the same individual on different days. Reproducibility and recovery were shown to be good.