Two measures of the degree of genome reorganization based on synteny data are presented. The first, q, measures how far the original syntenic relationships have decayed during the separation of two taxa. The second measure, Q, is a logarithmic transformation of q, so constructed that Q increases unlimitedly while q deflects when reorganization becomes substantial. Numerical examples are given for the genomic reorganizations that have occurred during the evolution of humans, mice and rats. The degree of genome reorganization between the mouse and the rat is estimated to be about 1/5 of the degree of reorganization between humans and the rodent species.