Despite clear functional significance, understanding of exploratory behavior is hampered by the lack of a careful descriptive account. This article reports extensive descriptions of rats' behavior in a stable environment and after environmental change. For 7 nights, 12 male Long-Evans rats (Rattus norvegicus) were given access to a large arena. Subjects budgeted session time differently across days, spending less time in the home cage and more time interacting with objects. Locomotion and general activities were typically unchanged over days. Several measures of object interaction showed systematic changes across sessions for the nonmanipulable object but not for the manipulable object. Finally, exploratory and spatial locomotor patterns were modified in response to the addition or removal of objects. These results indicate that exploratory and investigatory behaviors are lawful and orderly expressions of spontaneous behavioral organization.