ABSTRACT Prolonged stress hampers immune function and lessens disease resistance of fish, causing economic losses. Attention is thus been centered in the study of fish immunology. The main ‘in vivoʼ models used in immunological studies are: stimulation of immune response (by immunostimulant molecules); induction of inflammation; induction of immunosuppression by chronic stress; or administration of drugs. This trial aimed at evaluating existing protocols for immunosuppression by drugs in fish, adapted to slow release implants model, using hydrogenated vegetable fat (HVF), with the intention of set a controlled immunodeficiency state model for advanced studies. The implant model was not efficient in reducing the immune response in a controlled manner. Evidence of self, down-regulation in fish immune system was found in implanted fish, what should be further investigated using molecular tools.