The regular day-night changes in tissues, physiologic functions, and behavior of organisms are based on endogenous rhythmic processes which under constant conditions continue with periods slightly deviating from 24 h. These 'circadian' rhythms have properties of self-sustained oscillators. Under natural conditions, circadian rhythms are synchronized (entrained) to 24 h by periodic factors in the environment, the so-called 'zeitgebers'. In the laboratory, circadian rhythms can also be entrained to periods other than 24 h within certain limits. Data on the phase relationship between the circadian rhythm and an entraining light-dark cycle for vertebrates, insects, plants, and unicellular organisms are reviewed.

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Wild hares were trapped in the vicinity of Ottawa, Canada, and tested during summer and winter. After recovery from implantation of subcutaneous and substernal thermocouples, measurements of oxygen uptake and body temperatures were made during successive stepwise lowering of ambient temperature from 20 "C to -45 "C over about 5 hours. Measurements were also made during stepwise elevation of temperature from 12 'C t o 38 "C wer a 5-hour period and a final single test was made a t 40 "C,Hares trapped during the winrer had lower OF uptalre than did summercauqht hares at temperatures below thermal nt?urr,zliiy. The change was very similar i n magnitude to the seasonal c h a n~e i i~ instilation of the f u r (27%) Ermter i n winter). ' 1 he lower crilicnl tenlperattire IVRS sliiftcd from 4-10 DC in summer hares t o about -5 "C in winter hares.No seasonal differences were noted in substernal temperatures, but subcutaneous temperatures were significantly higher in winter-caught animals. Colonic temperatures were higher than substernal temperatures a t all ambient tem eratures.~r e c t r o l n~o~r a m s recorded frnrn the mid-hck showed that shivering WEIS graitcr in the cold drrrins summer thnn during winter in accortlance with the Irigher 0 2 consumption at any ~i v e n temperature, Shivering was a l w sliqhtlv Rrcater during sutnmer at the same lwel nf 0 1 consumption. V a q i n g harm showed a considerable tolerance to e1eu;ltion of temperature and a capaciiv to maintain npproximate cq~iaiity of body and ambient temperature at 40 "'c for somc timu through e i i~l i v e respiratory evaporative heat loss, which approxcherl 100'5 of heat production. Stepwise elevatior~ of ambient ternpernt~~re dld not reveal any seasonal tlificrencm in upper critical temperature (about 38 "C) or in clevatintl of lrorly Leniperature i n ~p i t e of differencm in ins~llation of the fur. !\ slightly greiiter proportion oI the ~o t a l heat loss was by evaporation during the summer.Caloric intake of captive hares outdoors was similar during winter and summer. I t is concluded that seasonal acclimatization in the varying hare is largely insulative with respect to cold and that changes in heat tolerance are minor lf present a t all. Insulative and behavioral modifications appear t o compensate for seasonal changes in temperature in the Ottawa area.While there is widespread interest in the phenomenon of acclimatization in mammals, there is still very limited information available on seasonal changes in thermal regulation occurring under natural conditions. In whitefooted mice (2) and in dump rats (3), there are indications that metabolic adjustments to winter may be similar in some respects to those observed in white rats exposed for several weeks to cold in the laboratory. The adjustments include increased cold resistance and greater heat production capability in 'Issued as N.R.C. No. 8498. 2Present address: