A 79-day rotational grazing experiment was conducted over the summer and autumn of 2007 to compare effects of grazing willow (Salix spp.) fodder blocks, a combination of small trees (i.e., 1.0m) and herbage, or perennial ryegrass (Lolium perenne)/white clover (Trifolium repens) control pasture on breath methane (CH 4 ) emissions, concentrations and solubility of CH 4 and sulphur hexafluoride (SF 6 ) tracer gas in blood, and haematology variables in young growing female sheep (i.e., hoggets). Measurements of gases in blood followed a double equilibration technique with two (n=20) replicate per treatment. Ten ewe hoggets in each replicate were dosed on day 22 with intraruminal slow release SF 6 capsules, an inorganic tracer gas used to calculate CH 4 emissions. Breath samples were collected over 5-day periods in weeks 5 (period 1) and 11 (period 2). Total condensed tannin (CT) concentrations calculated in the diet selected by the willow fodder block sheep was 12g CT kg/dry matter intake, with negligible amounts in control pasture hoggets. Compared to control pasture, grazing willow fodder blocks reduced CH 4 emission/kg metabolic body weight (BW 0.75 ) by 20% in period 1 (P<0.01), but not in period 2. Blood CH 4 concentrations (ng/mL blood) were similar for both groups on day 36, but higher (P<0.001) on day 76 for hoggets grazing willow fodder blocks, while a different trend was observed for SF 6 blood concentration being higher (P<0.01) on day 36 in hoggets grazing willow fodder blocks, but similar in both groups on day 76. Repeatability of blood CH 4 concentration was 75% in period versus 84% in period 2. Methane and SF 6 Ostwald solubility coefficients in blood were similar in both periods for sheep grazing willow fodder blocks and the control pasture. Hoggets grazing willow fodder blocks had lower BW gain (65g/day), carcass weight (16.1kg) and carcass fatness (9.2mm) than hoggets grazing control pasture (102g; 18.3kg; 11mm). Hoggets dosed with SF 6 capsules had lower (P<0.05) red blood cells, haemoglobin and haematocrit concentrations when grazing either willow fodder blocks or control pasture, while neutrophil (P=0.063), platelet (P=0.073) and monocyte (P=0.072), white blood cell and total lymphocyte counts (P<0.05) were higher for willow fodder block-fed hoggets than those fed the control pasture. Differences in the reduction in CH 4 emission between periods from grazing willow fodder blocks may be due to more willow leaf being eaten during the CH 4 measurement period in period 1 than in period 2.