Computer-based brief motivational interventions may be able to reach a high proportion of at-risk individuals and thus have potential for significant population impact. The present studies were conducted to determine the acceptability and preliminary efficacy of a computer-based brief motivational intervention (the motivation enhancement system, or MES). In Study 1, quantitative and qualitative feedback from 30 postpartum women and 17 women in treatment for drug use were used to modify the software. In Study 2, 50 urban postpartum women who reported drug use in the month before pregnancy completed the intervention and provided repeated within-session ratings of state motivation. In Study 3, 30 women were randomly assigned to intervention or control conditions with 1-month follow-up. Overall, women rated the MES as highly acceptable and easy to use and reported significant increases in state motivation at postintervention and at 1-month follow-up (d = .49). These preliminary results are encouraging and suggest that further work in this area is warranted.