Release of platelet antiheparin activity (platelet factor 4, PF4) induced by collagen and l-epinephrine, is normal in newborn infants without antenatal drug exposure. In contrast, PF4 release in platelets of infants of mothers with membrane-stabilizing drug ingestion is decreased to a greater extent than in their respective mothers. Platelet aggregation gives a qualitative indication of response. Platelets of mothers with antenatal drug exposure and those of their infants showed only a one-wave response to adenosine diphos-phate, while platelets of these same mothers still showed a two-wave aggregation response to l-epinephrine. In contrast, platelets of all neonates lacked epinephrine-induced aggregation response, even when washed and resuspended in adult plasma. Our studies show that prenatal drugs have an effect on the platelet release mechanism in both mothers and their exposed infants; but the effect on the epinephrine-induced platelet response is less in mothers. Newborn infants have different aggregation and platelet release response to epinephrine which suggests that platelet aggregation and the release reaction can occur independently of each other.