1. Ventilatory chemoreflex responses have been studied at rest during the recovery from a brief period of heavy exercise. 2. Six young, healthy male subjects each undertook four experimental studies. In each study measurements were made at rest during recovery from an exhaustive 1-2 min sprint on a bicycle ergometer with a workload of 400 W. Three levels of end-tidal 02 pressure (PO) were employed. Continuous ventilatory measurements were made during euoxia (end-tidal PO2, 100 Torr), hypoxia (end-tidal Po2, 50 Torr) and hyperoxia (end-tidal Po2, 300 Torr).Arterialized venous blood samples were drawn during each of the measurement periods for the estimation of arterial pH. In two of the studies, end-tidal CO2 pressure (Pco,) was maintained throughout at 1-2 Torr above the eucapnic level that existed prior to exercise (isocapnic post-exercise protocol, IPE). In the other two studies, end-tidal Pco2 was allowed to vary (poikilocapnic post-exercise protocol, PPE). Data from a previously published study on the same subjects involving an infusion of hydrochloric acid were used to provide control data with a varying level of metabolic acidosis, but with no prior exercise. 3. Ventilation-pH slopes in the IPE protocol were no different from control. Ventilation-pH slopes in the PPE protocol were significantly lower than in the IPE and control protocols (P < 0 05, ANOVA). This difference may be due to the progressive change in end-tidal PCO, in the PPE protocol compared with the constant end-tidal PCO2 in the IPE and control protocols.4. An arterial pH value of 7.35 was attained 30'4 + 2'7 min (mean+ S.E.M.) after the end of exercise in the IPE protocol and 17 1 + 1X4 min after the end of exercise in the PPE protocol. 5. Hypoxic sensitivities at an arterial pH of 7-35 were not significantly different between the IPE, PPE and control protocols (ANOVA). 6. Euoxic ventilation at an arterial pH of 7 35 was significantly greater than control for the IPE protocol (P < 0-001, Student's paired t test) and no different from control for the PPE protocol. 7. The results suggest that 30 min after heavy exercise, ventilation remains stimulated by processes other than the post-exercise metabolic acidosis, and that changes in peripheral chemoreflex sensitivity to hypoxia and acid are not implicated in this.