Infradian cycles of body weight occur in captive dormice,Glis glis. The period of these cycles is often about 2 months but is highly variable both between and within individuals. These cycles can persist for at least 3 years. This paper describes a number of variables that change on an infradian basis along with body weight. These include: food and water intake, weight of the liver, adrenal and salivary gland weight, testes weight and spermatogenic condition, body temperature and wheel-running activity. Both the amount of running and its distribution over a 24 h period change. However, infradian cycles do not appear to be generated by the circadian system because they persist even in continuous bright light sufficiently intense to disrupt circadian organization. They also persist in LD 18∶6, LD 12∶12, LD 6∶18 and in naturally changing photoperiods. No evidence of photoperiodic effects on cycles was detected except that animals in long days have slightly higher moulting scores than those in short days. Moulting is more marked when body weight stays relatively stable. In dormice remaining stable in weight for several months testes have sperm present. The period of infradian cycles is temperature dependent: in a cool room animals that hibernate have longer cycles with periods varying from a few months to about a year. In a number of respects at least, infradian cycles appear to be accelerated circannual cycles. The adaptive value of a system that gives rise to infradian cycles probably relates to opportunism in an unpredictable environment. Infradian cycles in dormice may be a useful preparation for obtaining data on spontaneous physiological and behavioural changes in hibernators in less than a year.