AbstractLittle information is available in which changes in the concentrations of total, protein, nonprotein, ammonium, nitrate:, free α‐amino, asparagine, and glutamine N in alfalfa (Medicago sativa L.) herbage have been measured at successive growth stages in the same investigation. In the current study, concentrations (% dry weight) of these N fractions were analyzed for the leaflets, stems, and total herbage of ‘Vernal’ alfalfa sampled when the spring growth in 1966 was 25‐, 41‐, and 53‐cm tall, and at first, 25%, and 75% flower.All N fractions decreased in percentage in the leaflets, stems, and total herbage with advance in maturity. Percentages of total N, protein N, and non‐protein N were higher in the leaflets than stems at all stages of growth, while percentages of ammonium N and glutamine N were similar in leaflets and stems. In contrast, percentages of nitrate N, free a‐amino N, and asparagine N were higher in the stems than leaflets. Percentage decreases in the concentration of N f.ractions in the total herbage from the youngest (25‐cm tall) to the oldest (75% flower) stage ranged from a low of 50% for protein N to a high of 75% for nitrate N.