The present paper describes the in vitro inhibition type, inhibition constant and inhibition rate of pig heart aconitase by aconitine. The inhibition of aconitase activity by aconitine is totally non-competitive. The datum of inhibition constant, Ki = 0.11 +/- 0.01 mmol/l, indicates high affinity of aconitine by aconitase. The rate constant of inhibition kl = 49 min-1, the half-life for aconitase inhibition of 1 min and the 50% inhibition time of 8 min by 0.1 mmol/l aconitine show high specific inhibition of the enzyme by the inhibitor. These results suggest a possible molecular reason for the toxic and pharmacological actions produced on experimental animals by aconitine.