We have used hydrothermal isometric tension (HIT) techniques in a sheep model to assess collagen crosslink stability and its contribution to the mechanical properties of the ovine thoracic aorta during perinatal and postnatal development. Aortic tissue was studied from fetal sheep, lambs, and adult sheep. Strips of tissue were loaded under isometric tension and heated to a 90
$^{\circ}$
C isotherm which was sustained for 3 hours. The decrease in load at this temperature is associated with collagen peptide bond hydrolysis and chain slippage, and the rate of this decrease is an inverse indicator of collagen crosslinking. The half‐time of load decay (
$t_{1/2}$
) was computed before and after tissue was treated with NaBH
$_{4}$
which stabilizes immature, reducible crosslinks. We observed a two‐fold increase in
$t_{1/2}$
of untreated tissue from the lamb to the adult, indicating that aortic collagen crosslinking increased during postnatal development. Furthermore, the
$t_{1/2}$
of NaBH
$_{4}$
‐stabilized lamb tissue was similar to that of the untreated adult tissue, suggesting that much of the immature crosslinking in the lamb is stabilized during postnatal development. These observations suggest (a) increased crosslinking occurs during postnatal development and (b) that this increase is largely due to a conversion of immature crosslinks into their mature, heat stable form.
