A theory to understand the total performance of complicated reproductive functions in mammals and to explain its integrative mechanism is presented with special reference to its mathematical formulation and digital computer simulation. Functional connections of components and subcomponents in the brain-hypophyseal-gonadal system are described through evaluation of experimentally established facts and our own hypotheses. Input-output relations of informations carried by neural spikes, 3 kinds of neurohormones and 3-5 kinds of hypophyseal and gonadal hormones are expressed in graphical, literal and mathematical terms. Sexual and species differences in both function and structure are also dealt with. The brain of both sexes is assumed to contain 3 different functional subcomponents such as neural, neurosecretory and store mechanisms. The neural mechanism is composed of detectors for photoperiodicity, copulatory stimulation and blood levels of hormones, and neurons which send either stimulatory or inhibitory signals to regulate the production and release of neurohormones. It is also assumed that each hormone can exert a positive feedback effect on the brain at first for a short period activating the stimulatory neural mechanism to be concerned, and then a negative effect for a longer period through the inhibitory mechanism. If the threshold, time constant and output level of each detector and neuron are adequately determined in response to neural or hormonal inputs, it is suggested that the secretory pattern of gonadotropins and sex hormones may be cyclic as in most female species and superficially non-cyclic as in the male. On the basis of such assumptions, a general principle for the regulatory mechanism is theoretically synthesized. This is exemplified by chronological changes in the dynamic state of the 4-day estrous cycle in the rat. This theory seems to be well evidenced by a result of numerical analyses by computer, showing a considerably good accordance with the performance of the real system.