Nonesterified fatty acids (NEFA) from C12 to C24 have been determined in sera of 13 young women at menstruation, and in the follicular and luteal phase of the menstrual cycle applying a new gas chromatographic microliter method. Neither the total NEFA content nor the concentrations of individual fatty acids showed significant differences in the three phases of the menstrual cycle. With the exception of stearic acid we found almost constant relative concentration values for the individual fatty acids within the medium range of the long-chain NEFA spectrum. Intraindividual andinterindividual dispersion was hardly greater than the variation caused by the method of determination itself, whereas the intra- and interindividual scattering of the rim fractions of the NEFA spectrum was much more distinct. The coefficients of variation for the total as well as individual NEFA content related to all examined women at any phase of the menstrual cycle were lower than 40 % with the exception of the rare erucic acid, behenic acid and lignoceric acid.