Age related cytochemical changes of thymic endocrine cells were studied in 78 day old chicks at five day interval to age of day 60 employing a panel of cytochemical stains. Methenamine silver revealed cell morphology including cell processes distinctly while diamine silver revealed a stronger argentaffinity in these cells. The cells had greater affinity for diamine silver compared to methenamine silver (Argentaffin), followed by formaldehyde induced auto-fluorescence, argyrophilia, lead hematoxylin and HCl-toluidine. The chromaffin reaction was the weakest. Cytochemically, three different endocrine cell populations i.e. argentaffin cells, argyrophilic cells and amine precursor uptake and decarboxylation series (APUD) /chromaffin cells, formed the resident population of thymic endocrine cells. Occurrence of numerous serotonin storing cells, moderately frequent APUD cells, and fewer chromaffin as well as mast cells suggests for a conspicuous reservoir of amine storing cells in thymus. Morphologically argentaffin cells were of four types i.e. the peripherally granulated spherical cells (Type-I), densely granulated oval cells (Type-II), pyramidal argentaffin cells (Type-III) and diffusely granulated elongated cells (Type-IV). The type-II argentaffin cells were most frequent in the medulla followed by the type-I cells and the type-III cells. The type-IV cells were least in frequency. The age related changes in frequency of these cells are also discussed.