[[4-Nitro-α-(4-oxo-2-thioxo-5-thiazolidinylidene)-o-tolyl]oxy]-acetic Acid is used in biological activities in the preparation and aldose reductase inhibitory activity of a new series of 5-[[2-(ω-carboxyalkoxy)aryl]methylene]-4-oxo-2-thioxothiazolidine derivatives.