<p>The executive functions are a suite of cognitive-control processes that enable goal-directed behaviour. Executive functioning is vital in allowing animals to adapt their behaviour to changing environmental conditions, which is a key determinant of the success of individuals, populations, and species under anthropogenic change. Despite this, very little is known about the mechanisms that underpin executive functioning in animals and how it manifests in behaviour. Moreover, whether and how executive functioning is impacted by environmentally realistic levels of chemical pollution is completely unknown, despite neuroactive pollutants, such as pharmaceuticals, having become widespread in ecosystems around the globe. We employed the Free Movement Pattern Y-maze to investigate how executive functioning manifests in decision-making and behaviour in a wild population of guppies (<i>Poecilia reticulata</i>), and whether this was disrupted by exposure to two environmentally realistic concentrations (9 ng/L or 309 ng/L) of the neuroactive pharmaceutical pollutant amitriptyline. We found that two of the three domains of executive function—working memory and cognitive flexibility—actively regulated cognition to inform behaviour during maze exploration. However, the role of executive functioning in directing navigation behaviour was not affected by exposure to amitriptyline. Given that neuroactive pharmaceuticals are increasingly detected in waterways, and executive functioning is critical for behaviours that dictate fitness, it is crucial that we better understand whether and how these contaminants affect executive functioning, and what implications this may have for wildlife living in human-impacted ecosystems.</p>