Rats placed into continuous darkness for 5 days maintained an “estrous cycle” pattern of HIOMT activity in the pineal gland, while exposure to continuous light for the same period of time abolished the pattern of HIOMT activity. Through the use of intermediate photoperiods, it was shown that the interaction of light and dark caused proportional changes in the mean rate of enzyme activity and specific changes in amplitude. By examining the effects of light on HIOMT activity in relation to specific terms of the mixed-wave equation the data demonstrated that the HIOMT rhythm in female rats is endogenous. A model of seasonal regulation of gonadotropin secretion by changes in pineal function is suggested. During the estrous cycle in the rat, the pattern of HIOMT activity in the pineal gland has been described as the sum of the oscillations of at least two rhythms whose frequencies differ slightly (Wallen and Yochim, 1974a). From this description a simplified mathematical model, using a mixed sine wave equation, was constructed and the temporal relationships between these rhythms and the 4- and 5-day estrous cycles were examined. In the equation, four major terms describe aspects of HIOMT activity. These are (a) “K,” a term which sets the mean rate of enzyme activity; (b) “A,” an amplitude term which describes the extent of oscillation around the mean rate of activity; (c) “es,” the period of a component rhythm to which we have assigned a dependency on photoperiod; and (d) “pa,” the period of a component rhythm which appears unrelated to photoperiod. In the present report, we have challenged one aspect of the mixed wave hypothesis by changing the duration of illumination to which the rats are exposed during each day. If the hypothesis is valid, then the equation should be applicable to all 24-h lighting regimes. The experiments described will reveal whether or not the component rhythms are endogenous and how the proportion of light per day affects the nonperiodic terms in the equation.