A prospective study was conducted to evaluate the effects of oral methadone on fetal activity (body movements, breathing, longest inactive period) for drug-dependent pregnant women on methadone maintenance. Seven consenting drug-dependent pregnant women between 26 and 37 weeks gestation were enrolled in the study. Pairs of ultrasound observation studies were conducted, before and after single-dose methadone (SDM) treatment and split-dose methadone treatment. There were significant decreases in both body movements (p less than 0.001) and breathing episodes (p less than 0.01), and a significant increase in the longest period of inactivity (p less than 0.001) following SDM. A similar but not significant trend was noted before and after split-dose methadone. The results of the single-dose studies differed significantly from normal controls. However, the results of the split-dose studies were similar to controls. It is recommended that women on methadone maintenance should be offered a split-dose treatment protocol.