Exercise-induced hypoxaemia (EIH) has been associated with an oxygen diffusion limitation. Because polyunsaturated fatty acids (PUFA) administration can modify cell membrane fluidity, we hypothesized that the importance of EIH could be reduced after a 6-week PUFA diet. Resting pulmonary functions and a maximal cycling test were performed before and after the diet, in eight master athletes -48 (SD 6 years)-. The partial pressure of O2 in arterial blood (PaO2), alveolar ventilation (VA) and ideal alveolar-arterial oxygen partial pressure difference (P(Ai-a) O2) were obtained at each exercise intensity. The extent of EIH at maximal exercise was significantly lower after PUFA [PaO2-17.2 (SEM 1.9) vs -12.9 (SEM 2.2)]. Before PUFA, VA accounted for 50% of the variance in the fall in P (Ai-a) for intensities below 80% maximal oxygen uptake (VO2max) and P(Ai-a)O2 for 60% between 70% and 100% VO2max. After PUFA, the reduction in EIH was highly correlated (r2 = 0.85; P < 0.001) to resulting changes in P(Ai-a)O2 and resting pulmonary diffusing capacity (DLCO)/VA but not with changes in ideal alveolar partial pressure of oxygen. The improvement in EIH following PUFA could be related to an increase in alveolar-arterial oxygen conductance following improved pulmonary diffusion.