Abstract: The localization of diaphorase activity of nicotinamide adenine dinucleotide phosphate (NADPH-d), acting as a marker of nitric oxide synthase (NOS), was studied in the thymus of rat. The NADPH-d active cells observed in the rat thymus were irregular with numerous projections and were located on the boundary of the cortex and the medulla. The thymus of the rat is innervated with parasympathetic and sympathetic nerve fibres. They are located as perivascular plexuses in the interlobular septa and on the corticomedullary border from where they extend to the organ parenchyma. The number and density of acetylcholinesterase (ACHE)-positive nerves increase up to the 3rd month of age and a reduction in thymic parenchyma and (ACHE)-positive nerves is visible in 12 month old rats. The density of adrenergic innervation increases up to the 3rd month and the adrenergic innervation of 12 month old rats corresponds to that observed in young rats.