Liver δ-aminolevulinic acid synthetase activity was measured in mice living under abnormal atmospheric pressure conditions for 15 h. In the group living under low atmospheric pressure (51 kPa) the enzymic activity, either basal or induced by starvation and/or allylisopropylacetamide, was significantly (p< 0.001) lower than that of the control group. In the group living under high atmospheric pressure (153 kPa) the enzymic activity was significantly (p< 0.001) higher than the one of the controls. Our results might possibly be explained by changes in the cellular redox state, the heme oxygenase activity or the serum erythropoietin levels.