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To understand the processes underpinning social decision-making, we need to determine how internal states respond to information gathered from the social environment. Brain monoamine neurotransmitters are key in the appraisal of the social environment and can reflect the internal state underlying behavioural responses to social stimuli. Here we determined the effects of conspecific partner cooperativeness during predator inspection on brain monoamine metabolic activity in Trinidadian guppies (Poecilia reticulata). We quantified the concentration of dopamine, serotonin and their metabolites across brain sections sampled immediately after ostensibly expe-riencing cooperation or defection from social partners whilst inspecting a predator model, using a familiar object as a control condition. Our results indicate dopaminergic and serotonergic activity differs with the coopera-tiveness experienced; these different neurotransmission profiles are likely to affect the expression and regulation of downstream behaviours that ultimately contribute to the patterning of cooperative interactions among in-dividuals in a population.