Platelet aggregation was studied in citrated whole blood by an electrical impedance method. Blood samples from normal volunteers were studied with the aim of finding a suitable method for the routine study of samples from patients. An erratic tracing and low maximum aggregation were seen in samples with a high normal haematocrit. Optimal aggregation was seen when blood was diluted to a haematocrit of .300; isotonic saline was a better diluent than platelet poor plasma. No appreciable differences were seen when the platelet count was diluted down to 50 X 10(9)/l, after which there was a progressive reduction in response. Dose response curves were obtained, and normal ranges for ADP, collagen, and sodium arachidonate were determined. Acetylsalicylic acid had a more pronounced effect on ADP aggregation than on collagen. Prostacyclin (Epoprostenol) and the synthetic prostacyclin analogue ZK 36,374 both showed dose dependent inhibition of aggregation, but the duration of effect of the latter was much longer (greater than 6 h).