The efficiency of photodynamic treatment with thiopyronine (TP) on the respiration and fermentation of yeast cells, on the metabolic oxygen uptake of isolated mitochondrial preparations from yeast, and on the enzyme activity of alcohol dehydrogenase (ADH) in vivo and in vitro were studied. Respiration was inactivated by treatment with TP and visible light more at the initial stage than was the fermentation. Isolated mitochondria and ADH in vitro were not as sensitive to treatment as were respiration and fermentation in vivo. Inactivation patterns of respiration and fermentation in vivo showed that the inactivation was not due to damage to the DNA. The hindrance of respiration and fermentation might be one of main causes of cell-death resulting from photodynamic treatment.