We have used hydrothermal isometric tension (HIT) tests to assess developmental changes in collagen crosslinking in the ovine thoracic aorta and pericardium. Tissue strips were loaded under isometric tension and heated to 90°C for 3 hrs. Load relaxation at this temperature is associated with collagen hydrolysis and chain slip-page, the half-time of load decay (t1/2) being an indicator of collagen crosslinking. t1/2 was computed before and after tissues were treated with NaBH4 which stabilizes immature crosslinks. The t1/2 of untreated tissue increased from the lamb to the adult, indicating that collagen crosslinking increased postnatally. Furthermore, the t1/2 of NaBH4-treated lamb tissue (3d pericardium and 21d aorta) was similar to that of untreated adult tissue, suggesting that much immature crosslinking in the lamb is stabilized postnatally. These observations suggest (i) increased crosslinking occurs postnatally, (ii) this increase is largely due to the conversion of immature crosslinks into their mature, heat stable form (Naimark et al., 1998, Wells et al., 1999).