The effect of perfusion in situ on the guinea pig placenta has been investigated using the tissue levels of lactate and pyruvate as biochemical indices. 2. Lactate levels in perfused placentae (10.22 mM) were significantly lower (P less than 0.05) than in non-perfused placentae taken from the same animal, i.e. internal controls (16.39 mM). The concentration of lactate is perfused placentae was similar to that in control placentae taken from animals immediately after induction of Nembutal anaesthesia. 3. It may be concluded that perfusion in situ prevents the significant rise in lactate concentration which is seen in paired-control placentae.