Nutrient measurements were performed during the Mediprod IV cruise of the R.V. Jean Charcot in the whole western basin of the Mediterranean Sea, special attention being paid to the Alboran Sea and to the Strait of Gibraltar. From these data, new values for the nutrient budget of the Mediterranean Sea are obtained. It appears that dissolved organic nitrogen (DON) contributes to the total nitrogen input in an amount of more than 50%, and that the river runoff contribution accounts for less than 10% of the total nutrient input. High values of the N : P ratio in the Mediterranean Sea (> 20) are explained by the nitrogen and phosphorus content of the superficial layers of the water inflow into the Mediterranean Sea, together with mineralization processes inside the basin itself. Below the deep water, a 100 to 400 m thick layer of bottom water shows chemical characteristics (lower nutrient concentrations and higher dissolved oxygen concentrations) which confirm its superficial origin.