Glutathione (GSH) is a tripeptide synthesised enzymatically from its components amino-acids by unicellular and multicellular organisms. GSH acts as a cellular anti-oxidant, protects the structural configuration of some enzymes, is involved in erythrocyte function and plays a role as co-enzyme in several reactions. We have found that GSH inhibits purified lactate dehydrogenase (1.56 U LDH/ml) from rabbit skeletal muscle after 6 min pre-incubation with an ED50 of about 5.4 microM. The inhibition is time dependent with a maximum after 45 minutes pre-incubation. Buffer (5 x 10(-2) M TRIZMA hydrochloride, pH 7.4) and a chelator (2 x 10(-3) M EDTA) in the pre-incubation solution did not prevent the inhibition. Prolonged dialysis was almost without effect on GSH-inhibited LDH activity solution, indicating either an irreversible or a very tight binding inhibition. Kinetic analysis showed that this inhibition is of a very tight binding and at the same time of the uncompetitive type. GSH also inhibits LDH activity of rat M. soleus and M. gastrocnemius homogenates. This effect is probably unrelated to the reducing property of GSH since dithioerythritol (0.17-1.34 mM) does not mimic it. Loading of MDCK cells with glutathione ethylester completely prevented the acidification induced by 2,4-dinitrophenol, suggesting that GSH may influence the glycolytic pathway in vivo.