This paper reports the results obtained in two pot trials and seven field experiments with potatoes, which were conducted from 1943 to 1947 and designed to study the effect of increasing rates of ammonium sulphate (20, 40, 60, 80, 120 and 160 kilograms of N per hectare) in the presence of PK (superphosphate and sulphate of potassium). The pot trials were carried out in the Central Experiment Station at Campinas and the field experiments were located on different areas of three sites of the State of São Paulo. Averaging the results obtained with all the doses the response to nitrogen corresponded to + 44% in the pot tests and varied from + 81 to - 29% in the field experiments. In the average of the latter the effect of nitrogen increased up to + 31 % when its rate of application was raised up to 60 kilograms, but decreased with the higher doses, falling down to + 2% with 160 kilograms. It was not possible to separate the experiments according to the soil types or locations. As the stands of most of them were damaged, the grouping according to the degree of injury showed that in those experiments whose stands were clearly impaired by the nitrogen applications, the highest response, about + 11%, was reached with 20-40 kilograms, the following doses depressing the yields so rapidly that the 160-kilogram rate caused a reduction of 49%; in the average of the experiments whose stands suffered no appreciable damage the highest response to nitrogen, +71%, was obtained with the 60-kilogram level, and the yields kept unchanged with further increments of the dose. The injury to the stands, as well os burning of the sprout and delaying in their emergence, were attributed to the usual method of application, which consists in the addition of the fertilizers (nitrogen, phosphorus and potash) to the furrows just before planting and slightly mixing them with the soil.