The use of psychoactive substances by offenders is a major preoccupation among correctional network clinicians, researchers and administrators. However, very little is known about this phenomenon during incarceration. The research presented here explores patterns of inmate drug use during imprisonment. Selected at random, 317 respondents in 10 Canadian penitentiaries were interviewed in 1999 to complete self-reported questionnaires. The data indicate various types of important changes, notably with regard to substances used, frequency of use, motivations for use, and inmates' perception of their psychological state during periods of intoxication.