Chronic methadone administration to female rats before, during and after gestation resulted in a delay or deficiency in brain development in their offspring compared with offspring of control rats. This deficiency in development was most noticeable as a decrease in protein and RNA content. A reduction in overall brain weight and DNA was less marked and paralleled a slight reduction in overall body weight. A comparison of the RNA/DNA and RNA/protein ratios suggested that the primary cause of the deficiency was associated with the RNA synthesis. Preliminary behavioral studies showed a significant decrease in ability of offspring of treated females to respond to a conditioning stimulus in order to avoid an electric shock. Even though upon gross observation the offspring from treated females seem normal, biochemical analysis and behavioral testing suggest that minimal brain deficiencies lasting weeks to months do occur.