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Free-running circadian rhythms of locomotor activity of Tenebrionid beetles Trigonoscelis gigas Reitt., taken from the Turkmenian sand desert, were monitored in DD. The effects of microgravity (mu G) - 11 days in space flight aboard the Russian BION-10 ’’COSMOS’’ satellite, and of 2G hypergravity seven days on a centrifuge, were determined.

Two kinds of effects were found.

In stable 2-peak records, there was a moderate decrease of tau in mu G and an increase of tau in 2G, both of about 0.3 hr.

In unstable records, alterations of gravity caused drastic deviations of tau and phi. Remarkably, two peaks of the activity rhythm, which are supposed to be controlled by separate oscillators, responded to gravity transitions in different ways.

Gravity effects on the circadian system could be explained from a direct effect on the oscillator(s) itself or from a feed-back by altered locomotion to the pacemaker.

Thus, for the first time the gravity dependence of a free-running circadian rhythm was proved in a combination of real space flight and centrifuge experiments.