1. Shivering is frequent during the post-anaesthetic recovery period, and there is no clear consensus about the best strategy for its treatment. We tested the efficacy of two commonly used analgesic drugs, pethidine and metamizol. 2. A randomized, double-blind, placebo-controlled clinical trial was performed, including 104 adult patients who presented with post-anaesthetic shivering during the recovery from general anaesthesia. They were randomized to receive placebo (n = 32), metamizol 25 mg kg-1 (n = 37), or pethidine 0.4 mg kg-1 (n = 35). The response to treatment was assessed 5, 15 and 45 min after drug administration, and the main outcome variable was complete suppression of shivering. 3. The efficacy at 5, 15 and 45 min was as follows: placebo 6%, 16% and 37%; metamizol 13.5%, 32% and 76%, and pethidine 89%, 91% and 89%. With both active drugs the efficacy at all three time intervals was significantly higher than that with placebo (P < 0.05). The differences (at 5 and 15, but not at 45 min) between pethidine and metamizol were statistically significant (P < 0.05). Both drugs were well tolerated. 4. The persistence of shivering at 45 min in two thirds of placebo-treated patients indicates that drug treatment is worthwhile; metamizol produces a better postanaesthetic shivering response than placebo, especially 15 and 45 min after drug administration; the efficacy of pethidine was the highest and the response to it appeared more quickly; however, at 45 min it was similar to that observed with metamizol. 5. Both metamizol and pethidine suppress postanaesthetic shivering, but the latter induces a quicker and more reliable response.