Summary: We obtain an approximation for the top Lyapunov exponent, the exponential growth rate, of the response of a single-well Kramers oscillator driven by either a multiplicative or an additive white-noise process. To this end, we consider the equations of motion as dissipative and noisy perturbations of a two-dimensional Hamiltonian system. A perturbation approach is used to obtain explicit expressions for the exponent in the presence of small intensity noise and small dissipation. We show analytically that the top Lyapunov exponent is positive, and for small values of noise intensity and dissipation \(\epsilon \) the exponent grows in proportion with \(\epsilon ^{1/3}\).