The objective of this part of the study was to determine biological value (BV) of pea protein for growing rats. Also, to compare the N-retention responses of the rats to supplementation of the pea protein with: (1) the same protein, (2) the mixture of essential and non-essential amino acids formulated to reproduce the pea protein, and (3) the mixture of essential amino acids only, formulated as above. The supplements were fed in the quantities defined by BV and equal to: (1-BV)/BV. A Polish cultivar of peas (Cyrkon), was a source of protein. Nutritional experiments were conducted using albino rats fed semi-purified diets and the N-balance method of Thomas-Mitchell, as described by Eggum [1973]. The relationship between increasing intake of pea protein and 24-h nitrogen retention in growing rats was linear only at the protein intake ranging from 0.50 g to 0.84 g/24 h. Pea protein supplementation with the same protein significantly increased (p