In the present study we investigate the effect of homocysteine (Hcy) administration, the main metabolite accumulating in homocystinuria, on butyrylcholinesterase (BuChE) activity in serum of rats. For the acute treatment, 29-day-old Wistar rats received one subcutaneous injection of Hcy (0.6 micromol/g) or saline (control) and were killed 1 h later. For the chronic treatment, Hcy was administered subcutaneously to rats from the 6th to the 28th day of life. Control rats received saline. The rats were killed 12 h after the last injection. In another set of experiments, rats were pretreated for one week with vitamins E and C or saline and 12 h after the last injection received one single injection of Hcy or saline, being killed 1 h later. Serum was used to determine BuChE activity. Our results showed that acute and chronic administration of Hcy significantly decreased BuChE activity. Furthermore, vitamins E and C per se did not alter BuChE activity, but prevented the reduction of this enzyme activity caused by acute administration of Hcy. The data suggest that the inhibitory effect of Hcy on BuChE activity is probably mediated by free radicals, since vitamins E and C administration prevented such effect.