Maternal dose--fetal teratogenic response data were obtained for a variety of narcotic and related compounds by single subcutaneous injections of the drugs into pregnant hamsters during the critical periods of central nervous system organogenesis. The number of abnormal fetuses from females injected with diacetylmorphine (heroin), thebaine, phenazocine, pentazocine, propoxyphene, and methadone increased as the maternal dose of the compounds was increased. By contrast, morphine, hydromorphone, and meperidine produced an increase in the number (per cent) of fetal anomalies only up to a certain maternal dose level. Further increases in maternal dose levels did not produce additional fetal anomalies. Comparative studies of single and multiple maternal doses indicated that diacetylmorphine (heroin) and methadone produced a four- to sixfold increase in fetal anomalies with repetitive doses whereas the percentage of malformed fetuses remained the same with hydromorphone (Dilaudid). The narcotic antagonists nalorphine, naloxone, levallophan, and cyclazocine blocked the teratogenic effects of both single and multiple doses of the narcotics.