Ruminants select nutritious feeds and avoid deleterious ones by relying on a learning process. We tested the hypothesis that they may similarly be able to select feeds for their medicinal properties, Le. to self-medicate. Our experimental model was lambs infected with gastrointestinal nematodes (GIN), and the temperate legume sainfoin (Onobrychis vicufolia) containing condensed tannins (CT). Parasitism by GIN is a major constraint of grazing livestock and interest is growing for bioactive forages with anthelminthic (AH) properties, as an alternative to chemotherapy. We predicted that parasitized lambs will express greater intake and preference for tannin-rich sainfoin than non-parasitized lambs, and that lambs will adjust sainfoin intake as a function of their parasitic status. We also tested the importance of a conditioning period in helping parasitized lambs to experience the beneficial effects of tannin-rich sainfoin. We used 40 ewe lambs and two types of sainfoin pellets with low (2.16%; T-) or moderate (4.06%; T+) CT contents. Lambs were split into two groups (n = 20), one being kept non-parasitized (NP) while the other was experimentally infected with 3000 larvae of the bloodsucking nematode Haemonchus conforms (parasitized, P). Preference tests (4-d long) between T+ and T- were performed before infection (Test 1), after parasite development (Test 2), after conditioning (21 day long period with only T + offered, Test 3), and after change in parasitic status (Test 4). Lambs' health status was monitored via faecal egg counts and haematocrit. We did not observe a self-medication behaviour as P and NP lambs expressed similar T + preferences in all tests (Group, P = 0.98; Group x Test, P = 0.92), and P lambs did not increase their T+ preference from Tests 1 to Test 2 (0.27 and 0.23 +/- 0.036, respectively). Initially, lambs expressed an aversion towards T + , but they switched to a preference after conditioning (Test 3:0.62 +/- 0.036), which was maintained after chemical deworming (Test 4: 0.66 +/- 0.059) (Test, P < 0.0001). Collectively, these results indicate that (1) the moderate CT content in T+ was sufficient to induce an initial sensorial aversion; (2) T + induced a positive reward in all lambs, whatever their parasitic status, which outweighed its potential AH effects; and (3) the conditioning period was important as it gave animals the opportunity to overcome their initial reluctance to consume a CT-containing forage, facilitating the learning process and allowing memorization of the sensorial and post-ingestive characteristics of the feed. Even if animals may not use temperate fodder legumes with moderate CT contents autonomously in a curative way, the absence of a trade-off between positive (medicinal, nutritional) and negative (toxicity of CT) effects may favour their use prophylactically, once animals have learned about their post-ingestive consequences.