Prof. Toshiharu Nagatsu began his research career in 1956 as a Ph.D. student working on catecholamines, a field that has determined his scientific life to this day. Soon after receiving his doctoral degree, he moved to the United States for postdoctoral research in the laboratory of Sidney Udenfriend. In 1964, he discovered tyrosine hydroxylase (TH), the limiting enzyme of catecholamines. Prof. Nagatsu continued the work on metabolism and its related enzymes in Nagoya; he developed a photometric assay for dopamine-β-hydroxylase, an enzyme that metabolizes dopamine to noradrenaline. In 1985, Prof. Nagatsu started studying the molecular biology of TH. He cloned and characterized the human TH gene and discovered four isoenzymes. He has been Chairman and Professor at Aichi-Gakuin University School of Dentistry, Nagoya, at the Tokyo Institute of Technology, Tokyo, at the Department of Biochemistry of Nagoya University School of Medicine, Nagoya, and he served as Professor and Director of the Institute for Comprehensive Medical Science of Fujita Health University, Toyoake. After he retired from Fujita Health University in 2001, he continued his work as Visiting Professor at the Department of Pharmacology, Fujita Health University School of Medicine, Toyoake, and at the Department of Brain Functions, Nagoya University Research Institute of Environmental Medicine, Nagoya.