This study examined the behavioral effects of prenatal and postnatal exposure to smoking, alcohol, and caffeinated beverages on 5-month-old infants. The sample consisted of 179 Caucasian infants and their mothers. All mothers were 19 years of age or older and had at least a tenth-grade education. Mental and motor portions of the Bayley Scales of Infant Development were administered to all infants. In addition, irritability or fussiness of the infant during testing was rated. Information regarding the mother's smoking and consumption of alcohol and beverages containing caffeine prior to and during pregnancy was obtained in an interview on the second day following delivery. Postnatal consumption data were obtained in a questionnaire mailed about 1 year after the 5-month assessment. Exposure measures for smoking, alcohol, and caffeine were calculated for the periods before, during, and following pregnancy, yielding nine independent variables. Outcome measures included the 5-month Bayley scores; the irritability rating; the infant's sensory, perceptual, and motoric exploration of the environment; and the mother's reports of infant sleep difficulties. Findings were consistent with the hypothesis that behavioral effects may be directly or indirectly attributable to smoking and use of caffeinated and alcoholic beverages at levels generally assumed to be harmless. (RH)