In mammals and birds, low oxygen levels in the lungs cause a constriction of the pulmonary vasculature. This response is locally mediated and is considered to be important for local matching of perfusion and ventilation. It is not known whether reptiles respond in a similar fashion. The present study describes the effects of altering lung oxygen levels (at a constant FCO2 of 0.03) on systemic and pulmonary blood flows and pressures in anaesthetised (Nembumal, 50 mg kg-1) and artificially ventilated turtles Trachemys scripta. During severe hypoxia (1.5-3 kPa PO2), pulmonary blood flow decreased in all animals; systemic blood flow increased, resulting in an increased net right-to-left shunt blood flow. The redistribution of blood flows was associated with reciprocal changes in the vascular resistances within the pulmonary and the systemic circulations (Rpul and Rsys, respectively). At 1.5 kPa O2, Rpul increased from 0.09 0.01 to 0.15 0.03 kPa ml-1 min kg during normoxia (means 1 s.e.m., N=5). Concurrently, Rsys tended to decrease from a normoxic value of 0.12 0.01 to 0.09 0.02 kPa ml-1 min kg (P=0.08). The effects of hypoxia on the haemodynamic variables persisted following atropinisation (1 mg kg-1) and cervical vagotomy, suggesting that the increased Rpul during hypoxia is locally mediated. This study therefore demonstrates that turtles exhibit hypoxic pulmonary vasoconstriction, although the threshold is low compared with that of mammals.