To determine the neurobehavioral consequences of alcohol use during different periods in gestation, we compared the behavior of 103 neonates born to women who: drank a mean of 12 ounces of absolute alcohol per week throughout pregnancy; drank a mean of 14 ounces of absolute alcohol and were otherwise comparable to the first group but stopped drinking in the second trimester, and never drank at all during pregnancy. Low socioeconomic status, predominantly black women applying for prenatal care at a large inner city hospital were recruited in the second trimester of pregnancy, and those reporting alcohol use were advised to stop drinking. Neurobehavioral evaluation with the Brazelton Neonatal Behavioral Assessment Scale was conducted at 3 days postnatal age. As a group, infants exposed to alcohol at any time during gestation were found to have significant alterations in reflexive behavior, less mature motor behavior, and an increased activity level in comparison to unexposed infants. Infants whose mothers stopped drinking in the second trimester were superior to those whose mothers continued to drink throughout pregnancy in observed state control, need for stimulation, motor tone, tremulousness, and asymmetries in reflexive behavior. These results indicate that characteristic damage does occur to the central nervous system of a fetus exposed to alcohol throughout pregnancy, and that exposure during only the early part of pregnancy also seems to have measurable effects. Multivariate analysis indicated that neither amount of alcohol used per week nor cigarette use contributed significantly to these effects on infant behavior.