The Nuffield Institute for Medical Research, University of Oxford, Oxford, OX3 9DS, U.K. Although a great deal is known about the metabolism of fetal tissues (Roux & Yoshioka, 1970; Jost & Picon, 1970; Stave, 1970; Greengard, 1971) there is no direct information about the influence the fetal metabolism exerts in the control of fetal growth. The assessment of any interrelationships has to be inferential, largely through an analysis of differences in fetal metabolism associated with variations in the fetal growth rate. There appear to be three major factors regulating fetal growth. The first is the maternal organism, by means of its control over the supply of nutrients and hormones to the fetus through its own metabolic activities and by its influence on the placental blood supply. The second is the placenta which may affect fetal growth through the production and transport of substances to the fetus. The third is fetal