In a rat model of volume-controlled hemorrhagic shock (mean arterial pressure = 20-24 mm Hg) causing the death of all saline-treated animals within 30 min, the i.v. bolus injection of ACTH-(1-24) (160 micrograms/kg) produced an almost complete and sustained reversal of the shock condition, with recovery of arterial blood pressure, pulse pressure and respiratory rate, and with 100% survival at the end of the experiment (2 h). The serotonin-depleting agent p-chlorophenylalanine (316 mg/kg i.p., administered 66-70 h before hemorrhage) almost completely prevented the effect of ACTH. The 5-HT1/5-HT2 receptor antagonist, methysergide, prevented the effect of ACTH completely when injected i.v. (5 mg/kg), but only in part when injected into a brain ventricle (i.c.v.) (15 micrograms/rat); the 5-HT2 antagonist, ketanserin, prevented the effect of ACTH completely when injected i.c.v. (1.5 micrograms/rat), but only in part when injected i.v. (0.5 mg/kg); the 5-HT3 antagonist, MDL 72222, largely prevented the effect of ACTH when injected i.c.v. (10 micrograms/rat), but had no influence at all when injected i.v. (3 mg/kg); finally, the 5-HT4 antagonist, GR 125487, had no effect when injected i.v. (5 micrograms/kg) or when injected i.c.v. (30 ng/rat). Overall, these data indicate that both CNS and peripheral serotonin play an important role in the complex mechanism of the ACTH-induced hemorrhagic shock reversal.