The study deals with the effect of artificial photoperiod on the interrenal and chromaffin tissues and physiological stress parameters in the teleost, Heteropneustes fossilis. Fishes were exposed to photoperiods of continuous illumination 24L:0D and continuous darkness 0L:24D for a short period (24hrs) and a long period (10 days) following which the histology of the tissues was carried out for morphometric measurements of the interrenal and chromaffin cells and blood was analyzed for the physiological stress parameters (plasma glucose, plasma chloride, plasma protein and N:L ratio). No changes in the physiological variables were observed following any of the treatments for short periods. Plasma glucose, plasma chloride and plasma protein levels increased significantly (p<0.05) following the 24L:0D exposures for ten days. Significantly the interrenal cells showed no noticeable change in size following any of the long term exposures whereas the chromaffin cells were found to be significantly (p<0.05) increased in size after the 0L:24D treatment for long period of ten days. The findings strongly indicate that long exposures to artificial photoperiod activates the chromaffin cells which possibly direct the stress responses. A serotonin mediated activation of the chromaffin tissue is suggested in this catfish known for exhibiting photoperiod dependence in several of its activities.