Tyrosine kinases mediate cellular signal transduction to endotoxin. A class of tyrosine kinase inhibitors, the tyrphostins, have been shown to protect mice from endotoxin-induced lethality. Neonatal rats and mice have been shown to be uniquely susceptible to lethal endotoxic shock. In our study, the effect of a lipophilic tyrphostin, AG 556, on endotoxin-induced neonatal and adult mortality andin vitroneonatal splenic cell thromboxane (TxB2), tumor necrosis factor-α (TNF-α), and nitric oxide (NO) production were examined. Neonatal rats (<24 h old) were administered tyrphostin (100 μg subcutaneous) 2 h before an approximate LD50dose ofSalmonella enteritidisendotoxin (.024 mg/kg/intracardiac). There was a significant decrease in mortality in the animals pretreated with 100 μg of tyrphostin (29% mortality in the treated group, n = 41 versus 53% in the vehicle control group, n = 40;p< .05). Also in adult rats tyrphostin (5 mg/kg intraperitoneal) 2 h before endotoxin (10 mg/kg intravenous) significantly improved survival (50% drug treated versus 84% in control, n = 12/group;p< .05). Adherent neonatal splenic cell mediator production of TxB2, TNF-α, and NO (measured by nitrite) in tyrphostin pretreated splenic cells were compared with endotoxin-stimulated splenic cellsin vitro. The studies (n = 4) demonstrate an increase (p< .05) in the production of TxB2, TNF-α, and NO in the endotoxin- (10 μg/mL) stimulated adherent splenic cells compared with basal. Tyrphostin pretreatment (10, 20, 50 μM) produced a dose-dependent decrease (p< .05) in endotoxin-stimulated TxB2and TNF-α production. NO production was not significantly reduced. In conclusion, tryphostin appears to have a protective effect on mortality in both adult and neonatal rat endotoxic shock. Tyrphostin decreased specific mediator production in stimulated neonatal cells. Thus, inhibition of signal transduction pathways of endotoxin activation by tyrosine kinase inhibition may provide an effective approach to treat endotoxic shock in the neonate.