Continuous monitoring of emitted carbon dioxide (VCO2) was taken as an index of respiratory exchanges in male Sprague-Dawley rats of different ages (31–814 days) submitted to a light (100 lx)-dark (LD) 06.00–18.00 h alternation at a temperature of 20 °C and with food and water ad libitum. In grouped 82- and 426-day-old rats harmonic and spectral analyses show that aging decreases the amplitudes of the circadian (τ = 24 h) respiratory rhythms, decreases the amplitudes and modifies the phases of the ultradian respiratory rhythms of mean and great (10 min < τ < 24 h) periods. Under the environmental conditions of the experiments, the techniques utilized show no statistically significant changes with aging for the circadian rhythms and only small changes in the ultradian respiratory frequ