The paper presents the results of an experimental activity aimed at investigating the statistical properties of the exposure to electromagnetic radiation generated by the GSM cellular system. Many channels radiated by sources at different distance from the measurement point were monitored for seven days to obtain long-term samples of the peak and average rms strength over six minutes. The purpose of the research is to shed new light on the time patterns and amplitude distribution of the exposure level, with the intent to study: (1) the relationship between peak and average strength so that the worst case exposure can related to the average exposure, which is the quantity that measurement standards focus on; (2) variations of exposure with traffic conditions.