Studies of marginal safe Se-intake have been carried out in a seleniferous section of China since 1985. Three areas with low, medium and high Se levels were selected for this study. The respective average daily Se-intake (mean +/- SE) was 70.5 +/- 4.8 micrograms, 194.7 +/- 22.9 micrograms and 1438.2 +/- 76.3 micrograms for males, and 62.0 +/- 3.6 micrograms, 198.1 +/- 23.8 micrograms and 1238.5 +/- 64.6 micrograms for females (average body weight: male 55 Kg, female 53 Kg). When the increasing rate of Se-intake was compared with the corresponding tissue-Se levels it was found that the whole blood Se-level reflected more closely the physiological range of Se-intake, while at higher Se-intakes it became less sensitive than the levels in hair, finger-nail and toe-nail, which were comparable to the sensitivity of urine. It is suggested that hair, finger- and toe-nail may all act as excretory organs when excess amounts of Se are ingested. Hair- and blood-Cd are somewhat higher in residents of the high Se area, but whether they have influenced human Se-metabolism at the high level of Se-intake is not yet known. Significant correlations on log-log plots were obtained between levels of daily Se-intake and whole blood r = 0.878), breast milk (r = 0.899) and 24-h-urine (r = 0.859). Highly significant correlations on log-log plots between levels of tissue were also obtained: urine Se--plasma Se (r = 0.968), whole blood Se--hair Se (r = 0.952), fingernail Se--toenail Se (r = 0.919), hair Se--fingernail Se (r = 0.914), hair Se--toenail Se (r = 0.891), whole blood Se--toenail Se (r = 0.849) and whole blood Se--fingernail Se (r = 0.836). The highly significant correlations found between the Se-intake and the tissue-Se level, and also between the Se levels of various tissues, could possibly conveniently be used to convert the known tissue-Se level to the corresponding Se-intake. Taken together with the wide range of Se-intakes and corresponding tissue-Se levels this would provide the necessary conditions for studying the marginal and maximal safe Se-intakes in humans.