Different biologic rhythms with effect on one and the same parameter may cause a complicated signal pattern in which no regularity can be visually identified. In this study an autocorrelation function approach, well established in the interpretation of electroencephalograms, was shown to be applicable also in the interpretation of processes with few points of observation. This technique, used on the excretion of estriol in urine, reveals several rhythms with wavelengths between 6 and 72 hours. Interference between these signals may explain the irregular visual appearance of plots of subsequent estriol determinations and the considerable rise and fall of urinary estriol excretion during pregnancy. The presence of two independent sources of the rhythms is suggested. It is concluded that the complicated and individual rhythmic changes in urinary excretion of estriol results in a low diagnostic specificity for fetoplacental malfunction of this component.