During the annual reproductive cycle, the pyloric ceca change in size and synthetic capacity. Changes in the amount of DNA indicated that seasonal changes in the size of the pyloric ceca resulted mainly from changes in the number of cells. The highest concentrations of RNA and the highest RNA:DNA ratios occurred during gametogenesis in the winter; lowest levels and ratios occurred in the summer or during environmental stress. Maximal activities of glucose-6-phosphate dehydrogenase (G-6-PDH), 6-phosphogluconate dehydrogenase (6-PGDH), glycerol phosphate dehydrogenase (GPDH), glyceraldehyde-3-phosphate dehydrogenase (G-3-PDH), and pyruvate kinase (PK) occurred during gametogenesis. Exposure of Luidia clathrata to different temperatures in the laboratory did not affect the level of activities of G-6-PDH, 6-PGDH, GPDH, and PK when measured at a constant temperature or concentrations of nucleic acids in the pyloric ceca. Activities of 6-PGDH, GPDH, and PK decreased, the concentration of DNA increased, and the concentration of RNA decreased in individuals exposed to low salinity. This resulted from decreased food intake and an apparent increase in the metabolic cost of cell-volume regulation. The significant effects of combinations of environmental factors indicate their importance when cellular responses during the reproductive cycle of L. clathrata are interpreted.