In preparation for large-scale investigations, we have conducted some methodological studies with small groups in order to determine patterns of stay and time activity patterns in various microenvironments at different times of the day. A diary technique was used. This paper reports some results of this methodological study: In one study we investigated a group of toddlers (n = 52, aged 2-3 years) attending a nursery school, and in a second study we compared two groups of medical students (n = 79 and 54, respectively). The reported data was analyzed by descriptive statistical methods. The toddlers spent an average of 87.6% of their time indoors (66.2% at home, 20.4% at nursery schools, 1% at other indoor areas), 11.2% of their time outdoors, and 1.2% of their time using various means of enclosed transportation. In the student study, time budgets of the two student groups differ only slightly. The students spent about 87% of their time indoors (about 66% at home, 14% in rooms connected with their study, 7% in other indoor areas), about 6% of their time using various means of enclosed transportation, and about 7% of their time outdoors. In order to describe the physical activity, the students were asked to evaluate their physical activity according to a low, medium or high physical strain. These evaluations were used to calculate a "mean activity coefficient" for each microenvironment and each hour of day. Not every individual in a group spent time in every given microenvironment during the period of investigation. Therefore, the mean over all the individuals of the group (the "group mean") and the mean over the individuals who in fact did spend time in any given microenvironment (the "actual doers mean") may be very different. There are two kinds of frequency distributions of time spent in various microenvironments: (1) The distribution of the individual's mean time spent in the microenvironment across the entire investigation period (distribution of the individual means) and (2) the distribution of the duration of time spent in the microenvironment for all participants at all days (distribution of the person-days). Both types of frequency distributions should be determined for the purpose of exposure and risk estimation for pollutants.