Abstract— Unconjugated bilirubin caused uncoupling of oxidative phosphorylation in brain mitochondria prepared from mature or weanling rats. Human serum albumin in an amount sufficient to bind 100 per cent of the added bilirubin was able to protect the mitochondria from the inhibitory effects of bilirubin. Bilirubin inhibited both the endogenous and DNP‐activated ATPase activities but had no effect upon the Mg2+‐stimulated ATPase; albumin prevented the inhibition. Copyright © 1968, Wiley Blackwell. All rights reserved