Sponges may regress in periods of environmental stress and form reduction bodies, or enter an overwintering phase. While the asexual production of gemmules is a response to long-term environmental changes; reduction is initiated in response to short-term unfavorable environmental events. Laboratory-reared specimens of S. lacustris were placed into either tapwater or heavily chlorinated swimming pool water in order to initiate the processes leading to reduction. Specimens were fixed at 0, 1, 6, 12, 24, 48 and 72 h to allow microscopical examination of the early stages of the degenerative process. Four trends are evident during the first 72 h leading toward reduction body formation. These are: a general withdrawal of the sponge into a central rounded mass; the genesis of lysosomal activity concomitant with phagocytic activity which increases noticeably throughout the period of observation cellular degeneration and decrease in the number of cell types in the sponge and initiation of connective tissue elaboration as cell mass decreases in proportion to an increasingly fibrous connective tissue matrix.