We determined the ratio of newly-synthesized type III collagen to the total of type I and type III collagen in mouse interparietal sutural tissue at selected ages between birth and adulthood (36 weeks old). We incubated mouse calvaria explants in Trowell-type organ culture dishes for one h and then added [14C]-glycine for two h. We dissected the interparietal sutural tissues for collagen solubilization by limited pepsin digestion. Fluorographic visualization of separated radiolabeled collagens, after SDS-PAGE, found the ratio of collagen type III a-chains to the total type I and type III a-chains to be age-dependent. The proportion of type III α -chains at birth was quite high, but there was a significant drop (p<0.05) during the first two days of life, probably because of the sudden environmental change from in utero. The proportion of type III α -chains rose significantly from day 2 to day 4, reaching a maximum and then dropping significantly to about the same proportion as at birth by day 7. A further significant drop took place during the second week of life, with the proportion stabilized at around 3.5% at two weeks to ten weeks of age. A final significant drop during the eleventh week of life led to no detectable synthesis of type III collagen after 12 weeks of age. The changes in the collagen phenotype ratio did not relate to changes in body weight during growth and development, which suggests that the interparietal suture may have an independent maturing pattern. Correlation of the ratio of newly-synthesized mouse interparietal sutural type III collagen with the growth rate of the rat interparietal suture (Massler and Schour, 1951) suggested a close relationship between high type III collagen ratios and periods of very rapid sutural growth. The desirable age for future orthopedic studies in Swiss male white mouse interparietal suture, in which collagen phenotype would be monitored, would be between four and ten weeks of age.