We have previously characterized two normotensive strains of rats which differ markedly in their susceptibility to spontaneous rupture of the internal elastic lamina (IEL), the Brown Norway (BN) being very susceptible and the Long Evans (LE) being resistant. Here we quantified biochemically the elastin and collagen content of aortae from adult male BN and LE rats aged 12,18 and 22 weeks and showed that the elastin content was lower and the collagen content higher in the BN strain than in the LE strain, resulting in a markedly lower elastin/collagen ratio in the former strain. These modifications were present both in the thoracic aorta, which is devoid of IEL ruptures, and in the abdominal segment where ruptures frequently occur in the BN rat, suggesting that they could represent a predisposing factor in the presence of other local factors. Quantifications of relevant mRNAs in aortae of younger male BN and LE rats by Northern blot showed that there are lower tropoelastin transcript levels in the BN rat at 6 weeks in both thoracic and abdominal segments than in the age-matched LE rat. In contrast there was no consistent interstrain difference in α<sub>1</sub> type I collagen transcripts and α<sub>1</sub> type III collagen transcripts were higher in the BN aorta only at 6 weeks in the abdominal segment. We conclude that the BN rat presents an aortic elastin deficit which appears to be in part explained by a decreased elastin synthesis in young, growing rats and may be genetically determined. However, a direct relation of this elastin deficit with susceptibility to rupture of the IEL cannot be concluded from this study.