We present a systematic study of the surface resistance of perovskite thin films and bulk samples at microwave frequencies. The d.c. resistivity rho of the various samples spans three orders of magnitude. For bulk samples, we obtain the usual rho(1/2) dependence of R(s). For rho < 1 mu Omega cm, R(s) displays conventional behavior, i.e. R(s) proportional to rho. However, for rho > 1 mu Omega cm, R(s) is not proportional to rho but to the conductivity sigma. Using this fact, we have made the first direct observation of a peak in sigma(1), just below T-c in a-axis films of YBa2Cu3O7.