The metabolic effects of low oxygen content on alcohol-dehydrogenase (ADH) activity and hemoglobin (Hb) concentration were investigated in IV-instar larvae of Chironomus riparius (Diptera: Chironomidae) from an Italian stream. Two series of short-term (48 h) experiments were carried out: exposure to (1) progressive hypoxia (95 to 5% of oxygen saturation) and (2) anoxia (at <5% of oxygen saturation). In (1), Hb amount increased with increasing oxygen depletion up to a critical value of oxygenation (about 70% of oxygen saturation). Below this percentage, the Hb amount declined to values comparable with those present in the control. The respiration rate (R) remained almost constant at oxygen saturation >50% and decreased significantly only after 48 h of treatment (= <5% of oxygen saturation) reaching values <100 mmolO2 gAFDW-1 h-1. ADH activity showed two phases of growth, within the first 14 h and over 18 h of exposure. Overall, we inferred that i) Hb might function as short-term oxygen storage, enabling animals to delay the on-set of anaerobiosis; and ii) alcoholic fermentation co-occurs for a short time with aerobic respiration, becoming the prevalent metabolic pathway below 5% of oxygen saturation (<1 mg L-1). These considerations were supported also by results from anoxia exposure (2). In such condition, larvae were visibly stressed, becoming immobile after few minutes of incubation, and ADH reached higher values than in the hypoxia treatment (2.03±0.15 UADH mg prot-1). Overall, this study showed a shift from aerobic to anaerobic activity in C. riparius larvae exposed to poorly oxygenated water with an associated alteration of ADH activity and the Hb amount. Such metabolites might be valid candidate biomarkers for the environmental monitoring of running waters.