Antibiotics are used extensively, but in addition to their anti-infectious effects some inhibit cellular energy metabolism. We investigated hypoxic tolerance following in vivo pretreatment with erythromycin and kanamycin, or in vitro pretreatment with ampicillin. Recovery of the CA1 population spike amplitude in hippocampal slices upon 15 min hypoxia improved time-dependently following single i.p. in vivo pretreatment with erythromycin (maximum at 6 h: recovery 90+/-7% (mean s.d.) vs 30% in untreated controls; p<0.01). The hypoxia-induced increase in NADH was smaller in slices that recovered from hypoxia. We conclude that antibiotics increase cellular hypoxic tolerance to a varying extent. Use of antibiotics in experimental studies may, therefore, distort conclusions about hypoxic sensitivity and confounding mechanisms. In contrast, antibiotics may provide an effective strategy to induce chemical preconditioning in humans.