Blood levels of 5-hydroxytryptamine (5-HT) and epinephrine are increased in conditions such as severe hypoxia, and hypovolemic or septic shock. The effect of these agents, which influence the blood coagulation system, were studied in vitro on platelets from newborns. 5-HT was determined in platelets from 18 newborns and 28 adults. The 5-HT content was 0.305 +/- 0.144 microgram/10(9) platelets in newborns and 0.675 +/- 0.212 microgram/10(9) platelets in the adult controls, a difference which is statistically significant (P less than 0.001). The 5-HT content of newborn platelets proved to be higher than that reported in the literature. Further studies on platelet aggregation due to 5-HT and epinephrine were then carried out. These demonstrated first-phase aggregation in 40% of cases when both 5-HT and epinephrine were applied simultaneously, while in the majority of the newborns no reaction was encountered when each of these substances was applied separately. However, in 2 cases, simultaneous application of 5-HT and epinephrine produced both first- and second-phase platelet aggregation. The reduced response of the platelets in the newborn to these aggregating agents can be considered as a protection against disseminated intravascular coagulation (DIC), particularly in high-risk situations.