A new model has been developed for the study of maternal phenylketonuria. Beginning on the 12th day of gestation the diet of pregnant rats was supplemented with 0.5% alpha-methylphenylalanine and 3% phenylalanine. This resulted in an 83% reduction of hepatic phenylalanine hydroxylase activity. The maternal plasma phenylalanine was elevated 10-20-fold for two-thirds of the day, but the degree and persistence of the fetal hyperphenylalaninemia may have been even greater. The brain phenylalanine concentrations in the fetus were raised up to 2900 nmole/g brain, whereas the highest level observed in the dam was 382 nmole/g. Experimentally-treated fetuses showed small reductions in both body and brain weight when compared to age-matched controls; however, no differences were seen in crown to rump length, litter size, DNA and protein concentrations per g, or in postnatal survival. Initiation of the diet at conception rather than on the 12th day caused a significantly greater inhibition of fetal growth, and 21% mortality. The fetal cerebral concentrations of methionine and the branched chain amino acids (valine, leucine and isoleucine) were decreased by hyperphenylalaninemia. From the 16th day on, the concentration of the inhibitor neurotransmitter glycine was elevated. Cerebral serotonin showed a 20-30% deficit and its primary metabolite 5-hydroxyindoleacetic acid a 71-77% deficit. Of twelve enzymes quantified in the brains of hyperphenylalaninemic fetuses only phosphoserine phosphatase showed any change. From the 20th to the 22nd day of gestation its activity was 46-67% higher in experimental than in normal fetuses. Measurement on the 22nd day of gestation showed that the increases in phosphoserine phosphatase activity and glycine content were present in brain stem, cerebellum, and forebrain.