The decomposition in rat serum of N-nitroso-(acetoxyalkyl)-alkylamines 14 C-labelled in different positions, was monitored. The determination of radioactivity of individual fractions permitted the determination of the rate-limiting step in their degradation scheme. From the primary nitroso-acetoxy-compounds the alkyldiazohydroxide shows the highest stability. Furthermore, it was shown that the a-hydroxy compounds of the secondary nitrosamines seem to be more stable than the hydroxylated primary ones.