Six field trials under irrigated conditions were carried out during 1991, 1992 and 1993, at Quilamapu Regional Research Center of the "Instituto de Investigaciones Agropecuarias" (INIA), Chillán, (360 31' S, 71 0 55' W, 217 m.a.s.I.), in order to evaluate the effect of basal and supplementary nitrogen applications on the grain yield of two semidwarf wheat varieties. A split plot design with four replications, two treatments and four subtreatments were used. Treatments: basal nitrogen aplications of 150 kg/ha. T1 =all nitrogen at sowing. T2=half nitrogen at sowing and half at end of tillering stage. Subtreatments: suplementary application rates of 50 kg/ha N in four development stages. t1 =during jointing. t2 =during heading. t3 =during flowering. t4 =during milk grain. The results indicated that splitting the nitrogen basal rate (50% at sowing and 50% at the end of tillering) generally increased grain yield, significantly, and that supplementary nitrogen application also increased grain yield significantly when this was applied at jointing stage. This tendency was more c1ear in winter than in spring wheat.