1. The analgesic potency of acetylsalicylic acid (ASA) is four times greater when administered intravenously than when administered orally. 2. The onset of the ASA analgesia after oral administration is significantly slower (30-60 min) than after intravenous (5-15 min) application. However, the duration of ASA-analgesia after oral administration is significantly longer (5 h) than after i.v. (2-4 h) application. 3. The onset and duration of ASA-analgesia in dogs after oral and i.v. administration cannot be correlated with plasma levels of ASA. During the period of analgesia, ASA can be detected only in extremely low concentrations, since it appears to be very rapidly hydrolysed to SA. 4. The development of an accurate and reproducible method for the separate determination of ASA and SA in plasma facilitated the direct correlation of plasma levels of these substances with ASA-induced analgesia.