Abstract
If guinea pigs were fed once in 24 hrs. a rise in the mitotic activity of adrenal cortex and thyroid gland took place, which as a rule reached a maxi mum somewhere between the 4th and 12th hr. following the feeding. This rise occurred irrespective of the time of day when the feeding took place. It was probably due to metabolic processes which are initiated by the intake of food. There was no indication in these expts. that light played a part in the detn. of rhythmic mitotic activity; but further studies are necessary to determine whether it may not be a subsidiary factor.