Three animal experiments were carried out to investigate the effect of condensed tannins (CT) on parasitic gastrointestinal (GI) nematode infections of sheep. Experiment 1 investigated the long-term effect of varying dietary levels (0–80 g/kg diet) of quebracho tannin (QT) on faecal egg counts (FEC) and feed consumption of lambs with experimental Haemonchus contortus infection. Dietary QT did not cause significant reductions in FEC although at high inclusions of QT there was an indication of a reduction in FEC. Some undesirable effects of tannins were also observed; feed consumption was significantly reduced (P<0·001) by dietary QT, while faecal water content increased significantly (P<0·01). In order to reduce the undesirable effects of dietary tannins in the host, short-term exposure to tannins was adopted. QT was administered as a drench, for 3 consecutive days, to growing lambs infected with H. contortus (Experiment 2) or its mixed infection with Trichostrongylus colubriformis (Experiment 3) and its effect on FEC, worm burdens and the GI tract was investigated. Administration of QT drench to sheep infected with the abomasal H. contortus alone significantly (P<0·05) reduced FEC and worm burdens (P<0·01) compared with the control group. In Experiment 3 the drench had little effect (P>0·05) on small intestinal T. colubriformis although similar effects to those seen in Experiment 2 on H. contortus were observed, suggesting that the location of the nematodes in the gut might be important. Apart from reducing feed consumption, the drench caused some physiological changes in the gut, particularly an increase in faecal water and mucus content, which were more marked following the drench than seen in the dietary inclusion trial. In conclusion, unlike dietary inclusion, QT worked more quickly and more effectively against H. contortus when administered as drench; T. colubriformis may require a higher dose or a prolonged exposure to the drench. The present findings suggest that it may be possible, perhaps with limited use of synthetic anthelmintics, to design appropriate feeding strategies using tannins to alleviate the problem of nematode infections in small ruminants. This may be particularly relevant to the tropics and subtropics where many species of plants contain high levels of tannins and where extracts high in tannins are readily available.