A greenhouse experiment was conducted to study the effects of nitrogen (N) forms (NO₃^–, 2.6 mmol/L; NH₄⁺, 2.6 mmol/L; NO₃^–, 1.0 mmol/L plus NH₄⁺, 1.6 mmol/L) and boron (B) (0.025, 0.5 and 1.0 mmol/L) on the growth and mineral composition of olive plants (Olea europaea L.). One-year-old own-rooted plants of the olive cultivars Megaritiki, Chondrolia Chalkidikis, Koroneiki and Kalamon were grown in plastic bags containing inert sand–perlite medium (1 : 1) and were irrigated with a 50% modified Hoagland nutrient solution. The plants treated with NO₃^– and 1.0 mmol B/L presented higher leaf B than those treated with NH₄⁺ and 1.0 mmol B/L, for all cultivars. The addition of ammonium form fertilisation caused a significant accumulation of B to root tissues when the latter consisted of 0.5 mmol/L. As B was increased in the NO₃^–-N treatments, the number of leaves and the root dry weight of cv. Megaritiki as well as the plant height of cvv. Koroneiki and Kalamon were reduced. The above results suggest that in olive, addition of NH₄⁺ to high B soils may be more beneficial for growth of olives than addition of NO₃^–. In most cases N form did not affect total leaf and root N levels. Also, the NH₄⁺-N treatments showed a decline in root phosphorus (P) and an increase in leaf potassium with increased B. The general trend was a decrease of P in leaves with increasing B.