This study was done to determine if chronic intermittent in utero exposure to morphine (MOR) during the last half of gestation results in hypoventilation and decreased ventilatory sensitivity to CO2 in the neonate. Pregnant guinea pigs were randomly assigned to once-daily treatment with saline and 1.5, 5.0, or 15.0 mg/kg MOR. Neonates were studied for 3 weeks. Prenatal exposure to 5.0 and 15.0 mg/kg MOR significantly increased neonatal minute ventilation and central inspiratory drive on day 7 while breathing room air or 5% CO2. The increase in minute ventilation was part of a withdrawal syndrome that included increased locomotor activity, but was not due to an increase in metabolic rate or sensitivity to CO2. We conclude that neonatal guinea pigs exposed once daily to MOR during the last half of gestation hyperventilate during the 1st week after birth. These changes are neither permanent nor followed by hypoventilation or depressed sensitivity to CO2.