Amitriptyline (AMI) and procainamide (PA) have been reported to inhibit the activity of human plasma butyrylcholinesterase, an enzyme important in the metabolic degradation of cocaine (COC) and its ethyl analogue cocaethylene (CE). Because both AMI and PA may be used in the treatment of COC intoxication and abuse, the effect of high pharmacological concentrations of these compounds on the degradation of COC and CE in pooled human serum was studied. AMI (1.8 micromol/L) modestly inhibited the degradation of COC by 4.2% and of CE by 4.0%. PA (42.5 micromol/L) profoundly inhibited degradation of COC by 42.7% and of CE by 47.2%. In contrast, lithium carbonate (1 mmol/L, control) showed no inhibition of degradation of either COC or CE. These results suggest that AMI and PA may prolong the half-life of COC and CE in human serum.