The influence of negative air ions on rectal temperature (Tr), heart rate (HR), oxygen uptake (VO2) and ventilation (VE) was examined in male subjects (n = 8) at rest and during two successive exercise bouts of 90 W and 180 W, each for 20 min on a cycle ergometer. Exposures at 4 different times of day (01:30, 10:00, 14:00 and 18:00 h) were presented to subjects under experimental and control conditions using a cross-over design. Results indicated that negative air ions significantly reduced resting values of all physiological variables (p between 0.05 and 0.01): these effects tended to disappear under exercise conditions, except for Tr. There was no significant effect of air ions on state anxiety pre- or post-exercise or on the perception of effort (p > 0.05). The significant circadian rhythm in Tr was reduced in amplitude by air ionisation although it retained its normal phase. Results confirm that negative air ions are biologically active and that they do affect the body's circadian rhythmicity.