The purpose of the present work is inquiring which kind of Christian judicial practice was increasing and developing during IV and V century, about matrimonial probibition between people in relationship, in third collateral degree and inter consobrinos, so we can see that tbe canonicallaw - shown by the Church fatber and the Councils -, which was stabilizing himself at the time, arrived at parenetic conclusions that met the Empires' legislation. The comparison with legal regulation, regarding the same field in IV century, enabled us to analyze the relationship between the two legislation, in order to cantest the doctrine, well known during the last decades, which presumes the derivation of the imperial latas from tbe Cristians' ones. This doctrine turns out not to be true, regarding, at least, prohibitions we examinated.