All 5 enzymes of the urea cycle have been determined on liver samples obtained from rats during late foetal and neonatal life. Since no argininosuccinate synthetase activity could be detected before birth it is suggested that the urea cycle does not function in the foetal rat. After birth, the activities of all the enzymes increase dramatically.

1. Highly purified rat mammary-gland acetyl-CoA carboxylase was inhibited by milk obtained from rats 12h after their young were weaned. 2. All the inhibitory activity was found in the particulate fraction (R(105)) obtained on centrifuging the milk. It could be extracted from milk fraction R(105) with acetone and identified as a complex mixture of non-esterified fatty acids, present in high concentration (nearly 10mm) in the milk. 3. Inhibition of acetyl-CoA carboxylase was observed at low concentrations (0.2-20mum) of several of these fatty acids when fresh fully active enzyme was used. Enzyme that had been partly inactivated by aging, or by storing in the absence of citrate, was stimulated by low concentrations but inhibited by high concentrations of fatty acids. 4. Various experiments suggested that fatty acids produce irreversible inactivation of acetyl-CoA carboxylase. 5. The effects of palmitoyl-CoA on mammary-gland acetyl-CoA carboxylase were found to resemble those of fatty acids, except that palmitoyl-CoA was effective at lower concentration. 6. The effect of milk fraction R(105) was tested on six other enzymes previously shown to decline to various extents after weaning. Although several of these enzymes were affected by unfractionated milk fraction R(105), none was significantly inhibited by the acetone extract or by low concentrations of lauric acid. 7. The findings are consistent, both qualitatively and quantitatively, with a regulatory mechanism whereby milk fatty acids shut off fatty acid synthesis in the mammary gland after weaning by inhibiting acetyl-CoA carboxylase.

In contrast to multiple injections of phenobarbital, a single injection of 50 mg/kg body wt of phenobarbital into immature male rats results, after a transient increase in hepatocyte mitotic activity, in a marked decrease in hepatocyte mitotic activity to below control levels by day 3, followed by a return to control levels by day 5. This unusual pattern of hepatocyte mitotic activity can be called fourth again by a second injection of 50 mg phenobarbital/kg body wt. However, a single injection of 50 mg/kg body wt of phenobarbital into immature male rats results in a pattern of change of liver wet weight, protein, and aminopyrine demethylase activity which is similar to that observed after multiple injections of phenobarbital, except that the changes are smaller in magnitude. Liver wet weight, protein, and aminopyrine demethylase activity increase and reach a peak within two days after phenobarbital injection, and then they return to control levels by five days. The same pattern of change in liver wet weight, protein, and aminopyrine demethylase activity can be elicited again by a second injection of 50 mg phenobarbital/kg body wt.