Sliding experiments have been carried out using copper surfaces in solid contact, and the friction traces have been analyzed statistically to study the spontaneous fluctuations in the friction force. The results suggest that the calculation of the standard deviation of the values of the instantaneous friction force can yield much information about the nature of the sliding process. High loads and smooth surface finish produce very steady traces for well-lubricated surfaces, while for unlubricated surfaces a rougher finish is required.