The present study is aimed at determining whether the induction of heat-shock protein (hsp) synthesis, heat-shock mRNAs, and tolerance development after arsenite application has been sensitized to low concentrations of arsenite in Reuber H35 rat hepatoma cells. Using a step-down arsenite treatment, consisting of a 1-hr pretreatment with 100 or 300 microM followed by an incubation with a lower concentration (1-10 microM), H35 cells were shown to exhibit increased sensitivity to low concentrations of sodium arsenite shortly after exposure to the high arsenite concentration, but not any longer when the low concentration was applied 4 hr after pretreatment. In this paper it is shown that exposure of H35 cells to sodium arsenite concentrations of 100 or 300 microM for 1 hr rapidly changes the sensitivity toward a second arsenite treatment with respect to the induction of the heat-shock response. It was observed that under conditions of enhanced sensitivity, an additional increase occurred in hsp synthesis as well as in hsp mRNA (as exemplified by hsp68 mRNA behavior) when low concentrations of arsenite were applied to arsenite pretreated cells. Since no effect of these low concentrations was observed in nonpretreated cells, the effect of step-down treatment results in a higher effect than could be expected based on summation. Furthermore, in sensitized cultures, cells are able to develop a higher level of tolerance in the presence of low concentrations of arsenite. It can be concluded that during a transient period of enhanced sensitivity, low concentrations of the original stressor are able to enhance hsp synthesis and to induce a higher level of tolerance in comparison with control cultures that are sensitized but not incubated in the presence of low concentrations of the original stressor.