It is well known that intra-uterine surgery for the introduction of indwelling catheters in utero* into the blood vessels of the fetal lamb causes some distutbance of homeostasis for a period of 5—7 days following the Operation (SHELLEY [22]). This includes nitrogen metabolism, for MELLOR and SLATER [18] have described a fall in concentration of total ot-amino N and urea in the allantoic fluid following surgery, together with a fall in the maternal plasma urea concentration. Observations of the changes in the fetal and maternal plasma free amino acid concentration following hysterotomy formed an important control in an investigation into the influence of an intravenous feeding Supplement «in utero' on fetal growth rate in which an infusion of amino acids and glucose was given into the fetal jugular vein for a period of 14 days (McFADYEN et al. [17]). The findings from the analysis of the plasma samples collected from the ewes and their fetuses during this investigation, are presented in this paper. An attempt has been made to relate them to the blood levels of urea and glucose observed. Total parenteral alimentation has been used previously in the adult sheep to study the amino acid metabolism in this species (TAO et al. [27]). Curriculum vitae