The influence of nitrate N supply on dry matter production, N content and symbiotic nitrogen fixation in soil-grown pea (Pisum sativum L.) was studied in a pot experiment by means of15N fertilizer dilution. In pea receiving no fertilizer N symbiotic nitrogen fixation, soil and seed-borne N contributed with 82, 13 and 5% of total plant N, respectively. The supply of low rates of nitrate fertilizer at sowing (“starter N”) increased the vegetative dry matter production, but not the seed yield significantly. Nitrogen fixation was not significantly decreased by the lower rates of nitrate but higher rates supplied at sowing reduced the nitrogen fixation considerably. Applying nitrate N at the flat pod growth stage increased the yield of seed dry matter and N about 30% compared to pea receiving no nitrate fertilizer. Symbiotic nitrogen fixation was reduced only about 11%, compared with unfertilized pea, by the lowest rate of nitrate at this application time. The pea very efficiently took up and assimilated the nitrate N supplied. The average fertilizer N recovery was 82%. The later the N was supplied the more efficiently it was recovered. When nitrate was supplied at the flat pod growth stage 88% was recovered, and 90% of this N was located in the seeds.