Twenty-four triplicate samples of the morning blood sugar of 17 fasting healthy women were taken from the cubital vein with a cannula at 15-sec intervals. The mean value was 4.44+/-0.69 mmol/l and the mean variation range 22.4+/-9.2%. Variability within the sets of three samples was 1.7+/-0.9%. In 23.5% of the cases, statistically significant periodicities (p less than 0.05) were detected by computing the periodogram. In 29.4%, several periodic components (2--4) were detected simultaneously. After deducting harmonic components, the residual scatter fell by an average 43.9%. In four cases the drop was significant in the F test at 1% and in one case at 5% level. The periodic components of the observed oscillations could be due to oscillation in feedback control circuits and the main role in the random component was evidently played by inadequate glucose blending in the circulation.