One hundred clients receiving methadone substitution treatment participated in an investigation of the relationship between methadone dose, time of daily self-administration and reported illicit heroin and other drug use. The study was conducted at two outpatient clinics operating from a single site at the Maudsley Hospital, London. Forty-seven per cent of clients reported using illicit heroin on one or more occasions in the week before interview. Multivariate analysis of methadone dose, time of methadone administration and duration of treatment indicated that the time of methadone administration was the most important predictor of illicit heroin use. Clients who took their methadone before 1500 h were less likely to use heroin than those who consumed methadone later in the day. This finding suggests the possibility of reducing illicit heroin use by methadone maintenance clients through the structuring of treatment programmes so that supervision of methadone consumption occurs during the early part of the day.