The purpose of these experiments was to determine the effects of oral caffeine ingestion on the kinetics of oxygen uptake (VO2), expired carbon dioxide output (VCO2), and expired ventilation (VE) in the transition from rest to exercise. Nine male subjects underwent three repetitions of constant load cycle ergometer exercise (approximately 80% ventilatory threshold) on two occasions. A single blind experimental procedure was followed, with one trial being performed 60 min after the subject ingested caffeine (7 mg X kg-1), while the second trial required the subject to ingest a placebo with the treatment order being counterbalanced. Ventilation and gas exchange were monitored by open circuit techniques. The data from three repetitions of the same protocol were pooled and modeled with a single-component exponential function incorporating a time delay: (formula; see text) Kinetic analysis of VO2 revealed no significant difference (P greater than 0.05) in the mean response time between treatments. In contrast, the mean response time for VCO2 and VE were longer (P less than 0.05) for the caffeine trial when compared to the control experiment. These data suggest that caffeine will influence both VCO2 and VE kinetics in the transition from rest to work and should be considered by investigators when planning experiments to study ventilatory and gas exchange kinetics during exercise.