Plasma 3-O-methyldopa (3OMD) concentrations in parkinsonian patients treated with levodopa on a long-term basis reflect daily levodopa dosage and do not vary markedly during the day. Oral challenges with 3OMD reduce the clinical response to levodopa infusions, but 3OMD is no more potent then phenylalanine in this regard. These observations, plus the fact that 3OMD makes a small contribution to the total concentration of large neutral amino acids competing with levodopa for transport at the blood-brain barrier, support the contention that 3OMD is not an important determinant of clinical response to levodopa.