BACKGROUND: Exposure therapy is a first-line treatment for anxiety disorders but remains ineffective in a large proportion of patients. A proposed mechanism of exposure involves inhibitory learning whereby the association between a stimulus and an aversive outcome is suppressed by a new association with an appetitive or neutral outcome. The blood pressure medication losartan augments fear extinction in rodents and may have similar synergistic effects on human exposure therapy, but the exact cognitive mechanisms underlying these effects remain unknown. METHODS: We used a reinforcement learning paradigm with compound rewards and punishments to test the prediction that losartan augments learning from appetitive relative to aversive outcomes. In a double-blind parallel design, healthy volunteers were randomly assigned to single-dose losartan (50 mg) (n = 28) versus placebo (n = 25). Participants then performed a reinforcement learning task, which simultaneously probes appetitive and aversive learning. Participant choice behavior was analyzed using both a standard reinforcement learning model and analysis of choice switching behavior. RESULTS: Losartan significantly reduced learning rates from aversive events (losses) when participants were first exposed to the novel task environment, while preserving learning from positive outcomes. The same effect was seen in choice switching behavior. CONCLUSIONS: This study shows that losartan enhances learning from positive relative to negative events. This effect may represent a computationally defined neurocognitive mechanism by which the drug could enhance the effect of exposure in clinical populations.