After multiple oral doses, the disposition rate constant (beta) of proscillaridin was studied in 4 young healthy volunteers and 33 elderly patients with congestive heart failure. Glycoside activity in plasma was assayed by the 86Rb-technique. In the volunteers the beta averaged 0.0299 corresponding to a half-life (t 1/2) of 23 h. beta could be determined in 24 patients and was 0.0139 +/- 0.0077 (mean +/- SD). The SD of beta due to biological factors was estimated to be 0.0072. The total variation of beta was 10fold. The mean beta corresponded to a t 1/2 of 49 h with a range from 19 to 209 h. It is concluded that the great variation of beta means difficulty in obtaining adequate plasma levels of proscillaridin and that a rapid elimination of the glycoside cannot be presumed.