In order to find ways of preventing or correcting the effects of desynchronosis, it is necessary to know the physiological mechanisms that are affected and to quantitatively determine their rate of recovery following a time-zone change. To best accomplish this, it is necessary not only to establish the rates of change brought about in performance and physiological systems during actual flight experiments, but to complement these observations with ground-based simulation experiments. A mathematical model was developed to quantitatively describe desynchronosis and was applied to data obtained from ground-based photoperiod shift studies using monkeys. An initial steady state, Vc, and a final steady state, Vs are postulated. The measured data vector, Vt, initially equals Vc, and finally equals Vs. The difference vector, Vts, with components A(t) and B(t), defined as the dot product and cross product of vectors Vt and Vs, is termed the desynchronosis vector. The trajectory of A(t) with time is given by: A(t) = A - e (alpha + betat), where A is the asymptote denoting complete resynchronization, alpha is proportional to the total desynchronosis on day O, and beta is the rate of resynchronization. The number of cycles required to achieve a 95% recovery, t95, is computed. This model has been applied to body temperature (BT) data from a monkey subjected to a 180 degrees phase-shift by alternating the photoperiod. The BT rhythm was initially stable and a 180 degrees reversal of phase with the new environment was eventually achieved. Estimated rephasal times were: 37% in 2.6 days; 50% in 5.6 days, and 95% in 8.4 days. Similar rates of internal and external resynchronization have been obtained from human photoperiod shift, ground-based experiments. Estimated rephasal time for BT rhythms with HR rhythms to the new photoperiod (t95) is 4.9 days.