Neonatal rats usually lose their marked tolerance to hyperoxia at about 1 mo of age. We examined the hypothesis that the marked dietary change that occurs at weaning might be important to this loss of O2 tolerance. We, therefore, prematurely weaned rat pups at 15-17 d of age, expecting to find an earlier loss of O2 tolerance. Surprisingly, the prematurely weaned rats showed consistently prolonged relative O2 tolerance compared with normally weaned rats at all ages tested from 35-85 d of life. For example, when challenged with greater than 95% O2 exposure for 7 d, the composite survival rate of the prematurely weaned rats (at 35-85 d of age) was nearly twice that of the normally weaned group (83 of 107 = 78% versus 44 of 107 = 41%, p less than 0.01). In the two experimental groups, nearly all comparative parameters examined were similar, including: 1) growth rate; 2) lung DNA, RNA, and protein; 3) lung antioxidant enzymes and enzyme responses to hyperoxia; 4) lung morphometry; and 5) lung elastin and collagen content. Only serum corticosterone and triiodothyronine levels differed considerably in the two groups. We conclude that premature weaning has a very marked and sustained positive effect on the relative retention of O2 tolerance in the growing rat.