1. Preparations of rabbit serum albumin, dialysed against rabbit plasma, were incubated at temperatures in the range 39–49°C at pH 7·3 and pH values in the range 7·2–8·0 at 43°C.
2. Denaturation of the rabbit albumin was studied by measuring (a) its susceptibility to tryptic hydrolysis and (b) catabolic rates in rabbits.
3. The rabbit albumin was demonstrated to denature by both criteria.
4. Assuming a similar denaturation to take place in vivo, it is possible to account for at least 10% of the normal turnover of albumin in rabbits.