Synopsis Female broiler chicks were offered semi‐purified diets containing crystalline essential amino acids, including proline, supplemented with various nitrogen sources. Individual final body weights, food consumption and food conversion efficiencies (FCE) were measured. In the first experiment 70 1‐d‐old chicks were given one of seven diets for 21 d; basal (1A) or basal plus: 12% l‐glutamic acid (1B), 17.05% monoammonium citrate (1C), 9.23% diammonium citrate (1D), 6.61% triammonium citrate (TAC) (1E), 2.45% urea (1F) or 4.05% triammonium phosphate and 12.56% calcium lactate (1G). Using body weight at 21 d and FGE as measures of the effectiveness of the various nitrogen sources the orders were 1E (171 g), 1B (154 g) > 1D (131 g), 1G (123 g), 1F (118 g) >basal (89 g) >1C (66 g), (P 1G (0.346), 1F (0.329), 1D (0.319) > basal (0.269) > 1C (0.192), (P < 0.05), respectively. In the second experiment 70 7‐d‐old chicks were offered one of seven diets for 14 d; basal (2A) o...