It is demonstrated that the absence of the natural day/light cycle has no principal effects on the development of the pineal gland during the first two weeks of postnatal ontogeny. Starting from the 21st day, functional activity of the pineal decreases, particularly under conditions of constant illumination. The shift of acrophase testifies to relative structural stability of the investigated rhythm. Key Words: pinealocytes; protein metabolism; circadian rhythmsEpiphysis is a unique organ whose structure and function are completely changed during the phylogeny of the vertebrates: from a photoreceptor organ to an neuroendocrine transducer [7,8].Some researchers regard the pineal gland in mammals as a circadian biological "clock". At the present time the oscillator of circadian biological rhythms in mammals is often associated with the suprachiasmatic nucleus of the hypothalamus [9,10]. However, anatomical and functional relationships between epiphysis and hypothalamic-hypophyseal complex and retina as well as circadian rhythms in the contents of various biologically active substances revealed in the epiphysis and strong dependence of these rhythms on illumination regimen suggest that together with suprachiasmatic nucleus the pineal gland plays an important role in the development and adaptation of circadian biological rhythms [2,3,5,6]. In this context a thorough study of histogenesis and of the effect of illumination regimen on biological rhythms of the epiphysis [ 1,4] is interesting.Our objective was to compare histogenesis and circadian rhythms of protein metabolism in pinealo-