Five groups of two volunteers lived during four weeks under arctic summer conditions (Spitsbergen) isolated in huts, without time information. They received lithium carbonate or placebo. All showed free-run of their circadian temperature and activity/rest rhythms with periods longer than 24 hours. Under lithium carbonate, a period-lengthening effect can be shown. This could help to understand theprophylactic effect of lithium salts in manic-depressive episodes.