A diurnal rhythm in pineal protein was found to occur in mature rats. In the male, a peak level is reached about 4 p.m., preceded by a gradual increase during the period of light and followed by a decrease that arrives at its nadir around midnight. The rhythm is shifted by a 4 h delay in peak and trough in mature female rats. Marked daily fluctuations were also recorded in pineal ribonucleic acid (RNA) of the mature male animals, a peak occurring around noon and a sharp nadir at midnight. No significant changes were noted in RNA levels of female rats, whether immature or adult, although a similar pattern in diurnal rhythm is apparent. While no diurnal changes in pineal deoxyribonucleic acid (DNA) were observed in either males or females, a trend towards higher values at midday and lower ones at midnight was characteristic here, too. The results obtained provide metabolic evidence for reported diurnal nuclear changes and indicate that cellular function is indeed related to nuclear size. The relationship between the diurnal rhythms of the RNA and protein of the pineal and its endocrinologically active indoles is discussed.