Tail vein blood was collected in heparinized capillary tubes immediately after a typical operant session. Serum was extracted with 3.4% trichloroacetic acid followed by a 5-minute centrifugation at 420 g, and assayed for ethanol content using the NAD-NADH enzyme spectrophotometric method (Weiss et al. 1993; Zapata, Gonzales & Shippenberg 2006). BECs were determined by using a standard calibration curve.
