Using mixed quantum–classical dynamics, the lowest part of the UV absorption spectrum and the first deactivation steps of keto-cytosine have been investigated. The spectrum shows several strong peaks, which mainly come from the S1 and S2 states, with minor contributions from the S3. The semiclassical trajectories, launched from these three states, clearly indicate that at least four states are involved in the relaxation of keto-cytosine to the ground state. Non-adiabatic transfer between the ππ* and nπ* excited states and deactivation via three-state conical intersections is observed in the very early stage of the dynamics. In less than 100 fs, a large amount of population is deactivated to the ground state via several mechanisms; some population remains trapped in the S2 state. The latter two events can be connected to the fs and ps transients observed experimentally.