Tansy ragwort (Senecio iacobea) is a widely distributed weed in the Pacific Northwest (U.S.A.) where it causes severe damage to the livestock industry. Sheep are highly resistant to toxic effects of ragwort, therefore, induction of ragwort preference in sheep was sought as a means to control this poisonous plant. The effect of two early exposure periods to tansy ragwort on future grazing preferences of five sheep genotypes was evaluated. Lambs were exposed at six months of age (ragwort fed fresh), and then at 15 months of age (ragwort fed as pellets). Four treatment groups were created; lambs exposed during both periods (T1), lambs exposed only during the first period (T2), lambs exposed only during the second period (T3), and lambs not exposed in either period (T4). Ragwort preferences were evaluated at 17 (Exp.l) and 19 (Exp. 2) months of age through direct grazing observation, and through plant observation before and after grazing. Experimental animals grazed in paddocks naturally infested with the weed. Results showed no treatment effects on preferences for tansy ragwort. In Exp.l consumption of the weed was so limited that comparisons were not possible between sheep genotypes nor between treatment groups. In Exp. 2 an interaction (p