Heat loss by radiation and conduction and oxygen release were measured on full thickness skin autografts in 46 guinea pigs on the seventh postoperative day. Using the same animals, the amount of hair on the graft was determined 3 months after surgery. This was assessed primarily as a measure of the extent of damage to hair follicles in the primary postoperative phase. Heat loss by conduction and oxygen release were also measured on the grafts and on symmetrically opposite control sites at this time. These measurements were used primarily as a measure of congestive hyperaemia in the subepidermal vascular plexus. Finally, the extent of changes in the collagen fibres was determined by histological examination. The author was unable to demonstrate any correlation between the circulatory picture on the seventh postoperative day and primary or secondary follicular damage. These results indicate that other, unknown factors have an important bearing on hair growth in grafts. These investigations also show that on the seventh postoperative day the reactivation of the pre-existing vascular system can be assessed mainly by heat loss by conduction and revascularization by new inward growth of vessels can be assessed mainly by the loss of radiant heat.