Abstract
Female rats which received 10 mg. of manganese per day above that in a stock ration 13.4 mg. per kilo were as successful in rearing their young as females receiving the stock ration only. The addition of MnSO4·4H2O equivalent to 2000 parts per million of ration did not retard the growth of rats over a period of 12 weeks immediately following weaning. Therefore, the slower growth of rats on a modified ration containing added manganese, which was reported in a previous publication 1, was not due to toxicity of this element but to a less suitable source of milk solids. Since under different conditions high levels of manganese may retard growth, no attempt is made to explain the contrary results of others by the data presented in this paper. The author wishes to express his thanks to Professors H. Steenbock and W. H. Peterson for their valuable suggestions.

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