ABSTRACT In the last few years our interest has been devoted to the energy metabolism of the eggs of the common toad Bufo arenarum Hensel which, like some other amphibian eggs, can cleave at a normal rate in the absence of oxygen or in the presence of cyanide (Barbieri & Legname, 1957). Under anaerobic conditions a rapid accumulation of lactic acid gives evidence of an intense glycolytic activity, which is inhibited when the eggs are returned to oxygen (Pasteur effect) (Barbieri & Salomón, 1963). Furthermore, an increase in oxygen uptake during the first 2 h of recovery has been observed (payment of the oxygen debt) (Legname, 1966). Taking into account the low value of the respiratory quotient (R.Q. = 0 ·5 –0 ·7) during this period it can be assumed that most of the oxygen is not involved in the oxidation of lactate (Legname, 1966). A similar respiratory response has already been described in Rana fusca (Brachet, 1934) and R.pipiens eggs (Cohen, 1955), and has served to support the idea of an ‘oxidative reserve’ in the eggs of these vertebrates (Brachet, 1934).