Objective: To evaluate the activities of Na+,K+-ATPase and Mg2+-ATPase in erythrocyte membranes from phenylketonuric (PKU) patients and to correlate the enzyme activities with their blood phenylalanine (Phe) levels, biogenic amines as well as with their precursors tyrosine (Tyr) and tryptophan (Try). Design and methods: Twenty three PKU patients were divided into group A (n = 12) on a stricted diet (Phe 1.57 ± 0.52 mg/dL or 0.10 ± 0.03 mM) and group B (n = 11) on a “loose” diet (Phe 24.45 ± 1.50 mg/dL or 1.72 ± 0.09 mM). The enzyme activities were measured spectrophotometrically, the amino acids with an automatic amino analyser and the biogenic amines with HPLC methods. Results: In group B, plasma amino acids (Tyr, Try), their biogenic amines [adrenaline (A), noradrenaline (NA), dopamine (DA) and serotonin (5HT)], (Na+,K+)-ATPase and Mg2+-ATPase activities were found remarkably decreased (p < 0.001). Conclusions: High Phe and/or low NA, DA, 5HT plasma levels may indirectly inhibit the erythrocyte membrane Na+,K+-ATPase and Mg2+-ATPase in PKU patients. The observed enzyme inhibitions could be a very informative peripheral marker as regards the neurotoxic Phe brain effects.