Exposure of pregnant rats to 25 R of x-irradiation on the fourteenth gestation day has produced in the male offspring an impairment of the development of the hepatic microsomal enzyme system which metabolizes hexobarbital. However, irradiation did not suppress the increase of enzyme activity brought about by the administration of chemical inducers (phenobarbital). Actinomycin, on the other hand, inhibited to varying degrees both the ontogenic and phenobarbital-induced increases in enzyme activity. The effects on the enzyme system have been supported by in vivo measurements of the duration of hexobarbital hypnosis. The ontogenic increase in enzyme activity is hormone-dependent, while that following phenobarbital administration is independent of hormonal regulation as evidenced by the response in hypophysectomized or sexually immature animals. It is concluded from these results that the inhibitory effect of x-irradiation on the hepatic enzyme system is mediated through an action on the hormonal regulation of enzyme activity. Evidence for this hypothesis is discussed.

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