Seasonal variations in environmental factors (irradiance, seawater concentration of NO,, NOp-and NH4+, temperature) and in physiological variables of Porphyra umbiljcalis (L.) Kiitzing (intracellular NOn-, NO2-and NH; concentrations, nitrate reductase activity, amino acids, soluble proteins, total C, total N, C:N:P ratio) were determined during winter 1991-92 at a site in Lagos (Malaga, southern coast of Spam). In the western Mediterranean Sea, the leafy thallus of P. urnbd~cahs occurs only during winter (mid-December to mid-March). Maximal % cover (80 %) was reached in the first part of February. Conditions of low temperature (14.7 _+ 0.6OC), short days (10 h daylight) and low global solar irradiance (2.5 to 3 4 kWh m-2) prevailed during the period of increase in ?/o cover. The period of decrease in % cover was characterized by long days (12 h), higher irradiance (3.4 to 5 kWh m-2), and slightly higher water temperature (15.5 f 1.34 "C). NO, concentration in seawater ([NO,],) at the study site ranged from 1 to 6.7 FM, [NOz-], was always lower than l pM, and [NH,'],, the largest source of inorganic nitrogen, ranged from 6.8 to 17.6 pM. Intracellular concentration of NO, ([NO,],) ranged from 2.8 to 51.3 mm01 1-' cell water (11.7 to 210.6 ymol g -' dry wt), which represents a concentration factor of up to 4.2 X 104 with respect to the seawater. [NO,-], ranged from 0.2 to 0.7 mm01 1-' cell water and [NH;], from 0.016 to 2.2 mm01 1-' cell water. Nitrate reductase (NR) activity varied widely during the winter cycle (between 1.2 and 9.4 pm01 NOp-g-' fresh wt h-'). It was found to be a very sensit.ive parameter for indicating when NOT is being used as the N source for growth. Therefore, low NR activity levels coincided with the highest [NO