We have investigated V - I characteristics in the microvolt region of c-axis-oriented films in the temperature range 20 - 70 K. A change in the curvature of the log V - log I curves was observed and to explain it we have used a model based on the Anderson - Kim theory of thermally activated flux creep for the washboard-type pinning potential in the presence of a pinning strength distribution. This model gives quantitative agreement with our experimental data.