The aetiology of sports anaemia is yet to be completely elucidated. Is it due to stress-induced fragility of red cells or performance-induced increase in plasma volume? In this study, the acute effects of endurance running on packed cell volume (PCV), haemoglobin concentration (Hb) and red cell count (RCC) were evaluated. Subjects were 20 female Physical and Health Education students at Lagos State University, Ojo, Nigeria. The subjects' mean age, body mass and height were 33.60+/-3.89 years, 64.40+/-11.50kg and 161.81+/-5.60cm, respectively. Standard procedures were used to determine the subjects' basal blood constituents, i.e. PCV, Hb and RCC. Pre-exercise haematological constituents were determined 24hrs before a 4.8km endurance run test (ERT), while post-exercise variables were measured immediately after the ERT. Results of the ERT, revealed that only two of our subjects were fit. Data were analysed using descriptive statistics and t-tests. The ERT resulted in significant increases of 1.7% and 0.57/dl in PCV and Hb, respectively while RCC was significantly reduced by 0.13x106mm. The 1.7% increase found in the subjects' post-exercise PCV was estimated to cause an 8.5% increase in plasma volume. The significant decrease in RCC was discussed in the light of the increase in plasma volume.