At least 25% of herbage grown in Britain contains Inadequate sodium (Na) for lactating cows (Baxter, 1968). The chances of defelcincy have been increased in recent years as more potassium (K) fertilizer has been applied which reduces herbage Na content. Voluntary DM intake of conserved feeds supplemented with Nacl is increased up to 5.5g Na/kgDM but thereafter declines (Shnelder et al., 1986). There is evidence that ruminants will select a high Na diet even when Na requirements are met (Michell and Moss, 1988). Milk yield responses to increased dietary sodium above ARC (1980) recommended allowances have been observed in some experiments (Mallonee, 1984; Schneider et al., 1988) but not in others (O'Connor et al, 1988). This experiment aimed to examine the effects of Nacl application to perennial ryegrass pasture on herbage and dairy cattle productivity, on the preference for Nacl fertilized pasture and to distinguish these effects from those of direct supplementation in concentrates.