Effects of progressive starvation of 12, 24, 48 and 60 h upon brain mitochondrial monoamine oxidase activity were studied. The enzyme activity was determined by three different substrates: 14C-labeled tryptamine, dopamine and kynuramine. With dopamine as substrate, the enzyme activity showed decline during 24 and 48 h of starvation. Monoamine oxidase when determined by tryptamine as the substrate, showed a decrease after 60 h of starvation. The use of kynuramine as substrate also produced a decrease in enzyme activity after 48 and 60 h of starvation. Refeeding the 60-h-starved rats for the following 24 h resulted in further decrease of monoamine oxidase activity of brain mitochondria from the 60 h starved values. The results suggest that oxidative deamination of biogenic amines is greatly inhibited during progressive starvation and remains low even after feeding the 60 h starved rats for 24 h.