Infant neonatal behaviour is significantly and differentially related to maternal marihuana, cigarette and alcohol use during pregnancy. Data on 250 babies born to healthy, predominantly middle-class women were analyzed using canonical analysis and multiple regression adjusting for potentially confounding variables. Prenatal marihuana exposure was associated with increased tremors and startles and poorer habituation to visual stimuli, prenatal cigarette exposure with increased tremors and poorer auditory habituation, whereas a relatively low level of alcohol consumption was marginally related to increased neonatal irritability.