-Bose-Einstein condensates loaded into kicked optical lattices can be treated as quantum kicked rotor systems. Noninteracting rotors show dynamical localization in momentum space. The experimentally tunable condensate interaction is included in a qualitative Gross-Pitaevskii type model based on two-body interactions. We observe strong and weak chaos regimes of wave packet spreading in momentum space. In the intermediate strong chaos regime the condensate energy grows as t 1/2 . In the asymptotic weak chaos case the growth crosses over into a t 1/3 law. The results do not depend on the details of the kicking.Introduction. -There is much interest in quantum behavior of systems that are chaotic in the classical limit. The hallmark model used in many such studies is the kicked rotor [1, 2], which was observed to exhibit a parametric transition from integrable dynamical behavior to a fully chaotic one. In physical terms, this corresponds to an unbounded diffusive increase of the rotor energy in the chaotic regime, as opposed to energies bounded by invariant tori in the integrable case. Once quantized, the kicked rotor disrespects the classical chaotic energy increase -a highly interesting phenomenon known as dynamical localization, similar to Anderson localization in disordered tight-binding models [3,4]. Dynamical localization has been observed in laboratory experiments with atoms in microwave fields [5,6], and in ultracold dilute gases condensed within kicked optical lattices [7][8][9]. In particular, the latter realization with a Bose-Einstein condensate, where the magnitude and the sign of the interactions between atoms can be tuned via Feshbach resonance by an additional magnetic field, has greatly opened opportunities to study dynamical systems in the presence of many-body interactions. In the mean-field approximation, these interactions can be modeled by adding a nonlinear cubic term in the Schrödinger equation, giving the well-known Gross-Pitaevskii equation. This can also be extended to the quantum kicked rotor model, result-