Figure 2. Time course of the antinociceptive effect expressed as percent maximum effect, observed with chronic infusion for 7 days of intrathecal morphine, MK801 10 nM/h, MK801 10 nM/h with morphine, and saline. Each line represents the mean+/-SE of five or more rats tested only on days 1, 7, and one day after discontinuation of infusion.