In these studies, platelet ageing during in vitro at 22 degrees C was compared with in vivo ageing using isotope labelling. Paired fresh and 5-d-stored platelets had a mean residual life-span (MRL) of 4.8 +/- 0.7 d and 3.2 +/- 0.9 d, respectively. After 2.1 +/- 0.4 d in vivo circulation, the MRL of the fresh platelets was equivalent to that of the 5-d-stored in vitro platelets. This suggests that platelet ageing for 5 d in vitro at 22 degrees C corresponds to 2.1 d in vivo ageing at 37 degrees C. Thus, the relative ageing at 22 degrees C in vitro was (2.1 d/5 d) = 0.42 of that at 37 degrees C in vivo. A similar ageing ratio (0.44) was obtained by measurement of the decrease in MRL during storage at 22 degrees C of platelets stored for 1, 5, 7, 10 and 14 d relative to the decrease in MRL of fresh platelets in vivo. ATP turnover rate at 22 degrees C was compared to the rate of 37 degrees C by measurement of the rates of platelet oxygen consumption and lactate production in vitro. In vitro ATP turnover at 22 degrees C versus 37 degrees C, was found to be 10.5 +/- 1.0 versus 21.6 +/- 1.4 mumol/10(12) plts/min, respectively. Thus, the ATP turnover ratio (0.48) at these two temperatures suggests that the relative decrease in ageing at 22 degrees C compared to 37 degrees C is similar to the relative decrease in metabolic rate at this temperature.