In an underground bunker built for the study of human circadian rhythms, one of the two experimental rooms is shielded against natural magnetic and electric fields. So far, autonomous rhythms of 82 subjects have been measured. As a result, the mean period value is lower in the non-shielded room than in the shielded room (significant with p< 0.01). Moreover, real internal desynchronization has been observed only in the shielded room (p = 0.0001); in opposition to this, apparent desynchronization with circa-bi-dian activity periods has been observed only in the non-shielded room (p = 0.01). This means that the total of the natural electromagnetic fields shortens the circadian period, and it strengthens the interaction between activity rhythm and the vegetative rhythms. Artificial constant fields, electric and magnetic, do not influence human circadian rhythms. However, a weak electric field, alternating with a frequency of 10 Hz, affects human circadian rhythms in the same manner as the total of the natural fields; i.e., it shortens the period (p< 0.001), and it prevents real internal desynchronization (p< 0.02). With this control of human circadian rhythms by a stimulus not perceptible consciously, a model for circadian rhythms derived from animal experiments can be confirmed in human experiments. On the other hand, with circadian rhythms as an indicator, natural electromagnetic fields are proved to be effective on human beings for the first time; this may be of interest with regard to space where these fields are absent.