Fulvic and humic acids extracted from a podzol were shown to stimulate respiration in rat liver mitochondria, when present at concentrations of between 40 and 360 mg L-1. Low molecular weight fractions induced a more significant increase in respiration than high molecular weight material. Humic substances, at concentrations of between 40 and 400 mg L-1, normally also increased the efficiency of the process of oxidative phosphorylation in vitro, particularly after contact periods with the mitochondria of over 1 hour.