The results of an extensive geographical survey of sodium and potassium values in topdressed ryegrass-white clover pastures clearly indicate that in many areas of New Zealand, particularly inland where the influence of sea spray is minimal, herbage sodium levels are below those needed for grazing animals. Moreover, as a direct result of the increased use of potassic fertilisers, particularly in the North Island, most pastures now have potassium levels well in excess of plant needs. Because potassium suppresses plant uptake of sodium and increases the leaching loss of sodium from the soil, it was concluded that excessive use of potassic fertilisers would be expected to aggravate or even create an animal-nutrition problem involving sodium. An additional animal-nutrition problem involving sodium is discussed in relation to the considerable differences measured in the extent to which the more important pasture and fodder species grown in New Zealand were able to absorb and translocate sodium from roots to shoots. In this respect plants can be classified into two distinct types according to the sites where sodium preferentially accumulates. Natrophilea: plants in which sodium readily accumulates in the leaf tissues. Natrophobes: plants in which sodium preferentially accumulates in the roots or lower stems with the result that only vary small quantities of this element are present in the leaf tissue. Because of this intrinsic physiological difference, natrophobes are unable to provide enough sodium for dietary needs of animals, even in a situation where there is enough sodium in the soil to produce satisfactory levels in natrophiles. In view of the depressing effects of potassium, the high risk of leaching under the humid weather conditions which prevail in New Zealand and the differences between plant species in their ability to translocate sodium, it seems that the most practical solution for correcting sodium deficiencies in grazing animals would be the direct supplementation with this element. Apart from the fundamental differences in the sites of sodium accumulation in natrophiles and natrophobes there are a number of other important features which further characterise these two distinct types of plant. In both natrophiles and natrophobes there is a significant relationship between sodium and nitrogen. What is more, both nitrate nitrogen and nitrate reductase activity, as measured by an in vivo method, were found to be predominately located in tissues where sodium readily accumulates. ln other words, nitrate reduction in natrophiles tends to occur in the aerial tissues whereas in natrophobes reduction occurs mainly in the roots and stems. Furthermore, it was concluded that the observed stimulation of nitrate reductase activity in tissues from both natrophiles and natrophobes, due to the addition of sodium chloride, was the result of an increase in nitrate nitrogen at the sites of reduction rather than any specific effect of sodium. However, the possibility that sodium has a stabilising effect on the nitrate reductase enzyme is discussed. From an ecological point of view it seems reasonable to suggest that natrophobes have developed their characteristic distribution of sodium and nitrate reductase activity, along with lower transpiration rates in order to tolerate more arid conditions than those which can be experienced by natrophilic plants.