We consider a class of observable, minimum phase single-input, single-output nonlinear systems evolving in R-n with relative degree rho and p uncertain differentiable time-varying parameters theta(t), belonging to a known compact set Omega subset of R-p, whose time derivatives (theta) over dot(t) are bounded, but are not restricted to be small or to have known bounds. We design a dynamic output feedback controller such that, for any initial condition: 1) all signals are bounded; 2) the effects of parameter uncertainties on the tracking error are arbitrarily attenuated; and 3) when (theta) over dot(t) is an element of L-1, asymptotic tracking is guaranteed with arbitrarily good transient performance. Adaptation may be switched off at any time, still retaining the closed-loop properties 1) and 2).