The three alcaloids inhibited the NADH oxidase system of electron transfer particles from beef heart up to 90--100 percent. The concentrations of half-inhibition amounted to 50 muM for berberine sulphate and tetrahydropalmatine and 0.55 mM for alpinigenine. All three compounds showed comparable inhibitions on the succinate-cytochrome c oxidoreductase system only at concentrations 20--25 times as high. The site of action may be the iron sulphur region of the complex I of the electron transfer system. The biological importance of this respiratory inhibitions should be taken into account.