In the existing works on the kinetics of renaturation of DNA, this process is always followed, in the course of time, by measuring the drop in U.V. absorbancy at 260 $m\mu$.This kind of measurement does not allow a separation of the contributions due to molecules in different stages of renaturation. In the present work on the contrary, the renaturation has been studied by means of the density spectra of the DNA solutions under examination, as obtained with the analytical ultracentrifuge in CsCl density gradient. It was thus possible to demonstrate that the first stage of the renaturation process (that in which two complementary strands come in contact) follows second order kinetics. Conditions necessary for obtaining reliable measurements of the activation energy for the renaturation process and for studying the kinetics of the second stage of the process (that in which the bonds between complementary base pairs are ordinately re-established along the two strands already in contact) are discussed and clarified.