Suppression of melatonin secretion in Ile-de-France rams by different light intensities

The intensity of cool, white, fluorescent light required to suppress melatonin secretion in Ile-de-France rams was investigated. Animals were conditioned to 12L:12D, lights on 0600 hours, 104 microW/cm2 (350 lux) at eye level and subjected to a 1-hour light pulse beginning 3 hours after lights off. Plasma melatonin measurements indicated that secretion was significantly suppressed by 0.30, 7.46, and 26.32 microW/cm2 (1.02, 25.10, and 88.60 lux, respectively) but not by 0.043 microW/cm2 (0.15 lux). A clear dose-response relationship was apparent between light intensity and degree of melatonin suppression.