The level of total nitrogen, protein nitrogen, nitrates (NO₃^-) and nitrites (NO₂^-) was measured in the leaves of kale of the cultivars Winterbor F₁, Redbor F₁ and Średnio Wysoki Zielony Kędzierzawy. The investigation was carried out in two successive years, the raw material being harvested three times each year, i.e. 10, 14 and 18 weeks after planting seedlings in the field. Depending on the year of the investigation, the cultivar and the date of harvest, the content found in 100 g fresh matter of kale was: 0.54-0.74 g total nitrogen and 0.46-0.50 g protein nitrogen; in 1000 g the content of nitrates (NO₃^-) was 248-2810 mg and of nitrites (NO₂^-) 0.14-0.95 mg. In both years the highest content of total nitrogen was found in leaves of Średnio Wysoki Zielony Kędzierzawy and of protein nitrogen in Winterbor F₁. The cultivar Redbor F₁ was characterized by the highest content of nitrates and nitrites. Comparing material from the three harvest dates, average values for the year and cultivar showed that the second harvest contained 9% more total nitrogen and 4% more protein nitrogen than the first, while the third harvest contained, respectively, 17% and 8% more than the first. Nitrate content fell by 67% and 83%, respectively, and nitrite increased by 5% (but later was reduced by 46%). Nitrate and nitrite are regulated in spinach and lettuce and for this reason understanding the accumulation of these compounds is critical if regulations are developed for kale.