The aim of this work was to establish to what extent the concentrations over which phenylarsine oxide (PAO) inhibits oxygen consumption and decreases cellular ATP content overlap with those used to inhibit protein internalization. The effects of PAO (1-100 microM) on 125I-labeled asialofetuin internalization, oxygen consumption, ATP content, and lactate dehydrogenase (LDH) latency of isolated rat hepatocytes were determined. Ten micromoles/liter PAO blocked 125I-asialofetuin internalization but had no effect on ATP content up to 20 min, only a slight inhibition (18%) on oxygen consumption, and no effect on LDH latency. Higher concentrations of PAO had increasingly deleterious effects on the parameters. These results show that higher concentrations of PAO severely affect the energy stores as well as integrity of the hepatocyte. We concluded that 1) use of PAO requires careful evaluation of its effects in each experimental preparation and 2) it should be possible to establish time and concentration parameters for use of PAO as an inhibitor of endocytosis while minimizing the influence of its other cellular actions.