Experiments were conducted with the aim of determining effects of melatonin on fluctuations in cloacal temperature (CT) and erythrocyte osmotic fragility (EOF) of layer hens. The CT of 14 melatonin-treated and 12 control laying hens were taken for 2 days, 1 week apart, at 06:00 h, 14:00 h and 18:00 h, using a standard digital thermometer inserted into the cloaca. The control birds were given water only, while the treated layers were individually administered at 18:00 h with melatonin orally. On day 12 of the administration, venous blood samples were collected from the layer hens to determine EOF. The overall mean CT values in melatonin-treated and control birds were 41.0 ± 0.05°C and 40.9 ± 0.04°C, respectively (P < 0.05). CT of the melatonin-treated group was lowest at 06:00 h (40.9 ± 0.10°C), but highest at 18:00 h (41.2 ± 0.05°C; P < 0.05). At 0.1%, EOF of 62.01 ± 3.75% obtained in melatonin-treated layer hens was lower (P < 0.001) than the corresponding value of 73.86 ± 2.59%, recorded in control layer hens. In conclusion, melatonin administration to layer hens sustained thermal homeostasis and decreased EOF in layer hens during the hot-dry season.

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