Field-based ecotoxicology studies are invaluable for uncovering the effects of contaminants of emerging concern (CECs) on aquatic organisms. However, large-scale exposures are still very rare due to prohibitive costs, the availability of replicated habitats, and the potential for exposure to cause lasting damage to the environment. Here, we evaluated the viability of internal slow-release implants as an alternative method for manipulating CEC exposures in aquatic wildlife using two fat-based carriers (coconut oil and vegetable shortening). We treated roach (Rutilus rutilus) with implants containing a high (50 mu g/g), low (25 mu g/g), or control (0 mu g/g) concentration of the behavior-modifying pharmaceutical oxazepam. We then measured oxazepam uptake in four tissues (plasma, muscle, liver, and the brain) over 1 month. The two carriers released oxazepam differently: coconut oil was the superior implant type because it delivered a more consistent dose across time, while vegetable shortening released oxazepam rapidly at the start of the exposure period. For both carriers and treatments, the brain and liver contained the most oxazepam. Overall, the method is a promising technique for controlled manipulations of pharmaceuticals in fish, and we have provided some of the first data on the suitability and contaminant release kinetics from different implant types.