Lambs are born with a well developed hypothalamo-pituitary-thyroid system and are producing tri-iodothyronine (T3) and thyroxine (T4) in significant quantities by birth. Thyroid hormones are high in the newborn lamb and appear to play an important role in adaptation, by the lamb, to the extra-uterine environment. The aims of this study were to look at the effects of the thyroid hormones T3 and T4, on the neonatal lamb's ability to cope with the cold and its activity and vigour. Secondly to obtain preliminary estimates of the heritability of the two hormone concentrations and the lambs rectal temperature to see if they could potentially be useful indicators of the lambs genetic ability to thrive.