Feedback inhibition of hepatic DNA synthesis. 
alpha-Hexachlorocyclohexane (alpha-HCH) and some other xenobiotic inducers were used to elicit adaptive increases in mono-oxygenase activity, size and DNA content of rat liver.  After elimination of the inducers, organ size and mono-oxygenase activity returned to normal whereas the DNA content of the liver remained increased.  Upon renewed treatment with an inducer the adaptive responses uncoupled.  While mono-oxygenase induction and liver enlargement did occur, DNA replication was largely suppressed.  These findings show that in the hyperplastic state the liver is resistant to stimulation of DNA synthesis by the inducers.  It is concluded that the DNA content of the liver (or the number of liver cells) is controlled by a feedback system which monitors an excess of DNA (cells) and suppresses cell replication if the content of DNA exceeds the normal level.  Organ mass has little, if any, effect on the operation of this feedback system.