1. Fourteen-week-old intact and castrated cockerels (White Leghorn) were injected intramuscularly with zinc (100 micrograms/kg body weight) as zinc ammonium sulphate solution once daily for 4 weeks and the effects on testes, pituitary and adrenal glands investigated histologically. 2. In intact cocks zinc decreased testes weight significantly, inhibited spermatogenesis and disturbed testicular hormone production. There was an increase in pituitary gonadotrophic cell activity, perhaps an indication of feed-back response to low concentration of testicular hormone. Cortical and medullary cells of the adrenal glands showed signs of activation. 3. No discernible effects of zinc injection on pituitary and adrenal cells of castrates were observed. This may reflect a decrease in pituitary and adrenal responsiveness to repeated stimulation with zinc. 4. The results indicate that the effectiveness of a given dose of zinc was dependent on the physiological condition of the cockerels and the effective site of zinc action was centred in the testes.

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