We describe the design and performance of a newly developed regulated environmental perfusion system (REPS). This system allows study of the effects of anoxia or hypoxia in cultured cells at physiological temperature, without the use of oxygen-scavenging compounds or metabolic inhibitors. The REPS incorporates a "canoe-shaped" flow-through chamber with access from above to allow positioning of pipettes for patch-clamp, microinjection, rapid-application perfusion, or microprobes for monitoring physical parameters. The combination of laminar flow and complete washout of perfusate within the chamber, and the use of a gas-tight perfusate delivery system and pressurized reservoirs containing media with pre-stabilized oxygen tensions (pO2 values) allow rapid production of accurate perfusate pO2 within the chamber. Perfusate pO2 in the chamber declined monoexponentially with time constants of </= 20 s to stable, pre-determined levels of 0 or 2 kPa (15 Torr). Shielding the gas/liquid interface of the chamber with an argon curtain only minimally decreased time constants at flow rates >/= 2 ml/min. The perfusion chamber of the REPS is easily mounted on the stage of an inverted microscope, for use with fluorescence imaging or electrophysiological studies of cultured cells. In tests with cultured rat cortical neurons, intracellular calcium concentration increased exponentially to values exceeding 1 microM during 10 min of anoxic insult, and returned to baseline values within 1 min after restoring normoxia.