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The effects of constant light, constant darkness, and diurnal lighting, in combination with pinealectomy or sham pinealectomy, on pituitary and plasma concentrations of radio-immunoassayable growth hormone (GH) were investigated in 23- to 24-day-old male rats. In addition, the effects on pituitary, accessory sex organ, testes, adrenal and body weights, as well as tail length, were measured. Three days after operation random groups of pinealectomized and sham-pinealectomized animals were placed in either continuous light, continuous darkness or diurnal light, and killed 28–29 days later. Analysis of the results revealed that the sham-pinealectomized group exposed to constant darkness differed from the other five groups by showing lower pituitary and plasma GH concentrations, body weight, tail length and accessory sex organ weight relative to body weight and testes weight. No statistically significant differences were found in pituitary or adrenal weights. It is concluded that increased pineal function occurring in darkness probably inhibits secretion of GH releasing factor, decreasing pituitary synthesis and release of GH.