Objectives We Studied the vascular effects of betamethasone (BM) and/or tracheal occlusion (TO) in fetal rabbits with surgically induced congenital diaphragmatic hernia (CDH). Methods At day 23 (pseudoglandular phase; term = 31 d). 54 ovarian-end fetuses from 27 does underwent induction of CDH. Thirteen did receive either 0.05 mg/kg BM, on days 28 and 29 with a 24-h interval. or 14 saline [controls (CTR)]. At clay 28, one ovarian-end fetus Underwent TO and harvesting was at term. In total, we compared (ANOVA) lung-to-body weight ratio (LBWR) and vascular morphometric indices in survivors from the following groups (n - number alive at delivery): CDH (9); CDH + TO (10); unoperated controls (14); CDH + BM (10): CDH + TO (9): controls CTR + BM (13). Results Maternal BM had no effect on LBWR. LBWR was comparable to normal in CDH fetuses undergoing TO. Both TO and BM have an effect on medial thickening due to CDH which is larger when both interventions are combined. Conclusions Both TO and BM lessen peripheric muscularization present in CDH lungs and their effect is cumulative. Copyright (C) 2009 John Wiley & Sons, Ltd.