A freshwater teleost, Oryzias latipes, responded to darkness with paling of the skin, melanosome aggregation within melanophores. The response took about 24 hr to become equilibrated, intermediate tones, being different from the background response in light in the time-course of the response. Eye removal, hypophysectomy and pinealectomy did not affect this response to darkness. Spinal sectioned and chemically sympathectomized fishes failed to respond to darkness, remaining dark in darkness. Dibenamine-treated animals did not become pale in darkness. These results suggest that the sympathetic nerves play a primary role in the response to darkness.