Scopolamine (0.1 - 3 mM) did not produce behavioral sensitization. Planarians pretreated twice with scopolamine (0.1, 1, 3 mM) on day 1 (60 min apart) underwent 2 days of withdrawal before being challenged with scopolamine (0.1, 1, 3 mM) on day 4 (scopolamine/scopolamine groups). Control planarians for each respective scopolamine concentration were pretreated twice with water on day 1 and then exposed to scopolamine (0.1, 1, 3 mM) on day 4 (water/scopolamine groups). Data are expressed as mean activity counts ± S.E.M. during 1 min of scopolamine exposure on day 4. Insets) Data are expressed as mean activity counts ± S.E.M. in planarians exposed to scopolamine (0.1, 1, 3 mM) for 1 min on days 1 (twice) and 4. *p < 0.05 versus day 1 exposure. N = 8 planarians per group.