The lung of rats is not fully developed at birth and complex structural processes termed "secondary alveolarization" take place within the first 3 weeks after birth. To assess how lung development affects breathing male WKY rats at post natal days 7, 14, 21, 30, and 90 were subjected to lung function tests. Total lung capacity (TLC) increased from 1.35 ± 0.11 ml (day 7, mean ± SD), to 1.69 ± 0.05 ml (day 14), to 2.28 ± 0.25 m (day 21), and to 16.75 ± 0,95 ml at day 90. Tidal volume changed proportionally to TLC (0.15 ± 0,02 vs. 2.5 ± 0.38 ml in 7 and 90 days old rats) while respiratory rate decreased from 250 ± 15 1/min to 177 ± 14 1/min from day 7 to 90. Minute ventilation continously increased from 37.5 ± 6.6. ml/min (day 7) to 411.2 ± 74.3 ml/min in 90 days old rats. Ventilation related to body weight decreased with age and was 1.72 ± 0.38 and 0.98 ± 0.17 ml/min/g in 7 and 90 days old rats, respectively. This study shows that breathing pattern and lung size changes substantiatially in the developing lung and it provides basic information to the scientific communtiy, e.g. for use in particle exposue studies. Supported by NIH: HL070542