It is well known that hemodialysis (HD) causes a rise in plasma tissue-type plasminogen activator (t-PA). Although there have been several suggested mechanisms responsible for this effect of HD, the precise cause has not been well understood yet. Another complication of HD, when performed with acetate-containing dialysate, is hypoxemia, which is commonly observed during the first hour of the session. The purpose of this study was to investigate the relationship between dialysis hypoxemia and HD-induced t-PA changes during the first two hours of HD. HD caused significant increase in plasma t-PA antigen levels. When individual t-PA profiles versus time were examined, two patterns were observed. Whilst ten subjects (%56) experienced minimal or no increase, t-PA antigen level of the remaining eight subjects began to rise at 30 minutes and continued at that level up to 90 minutes, when the last samples were drawn. The courses of pO2 were also different; whilst the former group had "early-onset and short-term" hypoxemia, the latter had "late-onset and prolonged" hypoxemia. The amount of increase in t-PA antigen and the amount of decrease in pO2 were correlated at 60 and 90 minutes of the HD session. Thus, it is concluded that dialysis hypoxemia may contribute to HD-induced rise in plasma t-PA levels. Further studies comparing different dialysates and dialyser membranes are required to confirm this hypothesis.