The previous chronoamperometric method described for the determination of lecithin in ethanolic-aqueous medium based on the inhibition of the oxygen reduction process at the dropping mercury electrode is applied to the determination of lecithin and sphingomyelin in methanolic-aqueous solution. The method permits the analysis of these phospholipids in isolation, or the sum of them when they are found in mixture, in the 4–40 μg/ml range (20–200 pg of phospholipids) with a standard deviation of 1%. The effect of the surface-active substances present in amniotic fluid on the inhibition of the oxygen reduction process is similar to the lecithin and sphingomyelin commercial samples giving directly chronoamperometric curves suitable for a quantitative evaluation of surfactants in amniotic fluid.