In a series of experiments the effects of p-chloroamphetamine (PCA) on shock-elicited aggression in rats were investigated. 15 min after 5 mg/kg PCA, shock elicited aggression was inhibited. 2 h to 4 weeks after PCA, fighting was facilitated. Both the inhibitory and the excitatory effects of PCA were directly related to the dose of PCA (1.5, 2.5 OR 5 mg/kg) and were blocked by pretreatment with p-chlorophenylalanine but not by alpha-methyl-p-tyrosine. PCA-increased pain thresholds 15 min after injection and then decreased pain thresholds over the next 24 h but not thereafter, even though shock-elicited aggression continued to be facilitated. The results are consistent with the idea that inhibition of shock-elicited aggression is associated with enhanced release of serotonin whereas enhancement of shock-elicited aggression is associated with serotonin depletion.