Seasonal variations in tissue nitrogen, carbon, amino acids and ammonium were determined for the brown algae Macrocystis integrifolia Bory and Nereocystis luetkeana (Mertens) Pastels and Ruprecht, For M. integrifolia, the proportions of tissue nitrogen and carbon in blades, bulbs and stipes were also determined. The composition of the two algae in terms of the above constituents was similar. In addition, ammonium, nitrogen and protein-bound amino acids showed distinct seasonal trends with high values during the winter and low levels during the summer. The range for nitrogen was 0.8–3.0% and for proteins 7.6–11.7% of dry weight. In contrast, carbon content and C/N ratio showed the reverse trend with higher values during the summer and lower values during the winter. The range for carbon was 19–31% of dry weight, and the C/N ratio showed a range of 9–37. The free amino acids did not show any specific seasonably. Tissue nitrogen and carbon showed higher values in the blades than in the bulbs and stipes.