Serum chemical data are presented for forty-five lions, Panthera leo, living in the Etosha National Park, Namibia. Most results are similar to those for captive lions. The effects on blood assays of twenty-one environmental, physiological and behavioural variables are investigated. Age and immobilization-related effects were the main variables operating within the normal range. The major finding related to behavioural variables associated nomadic individuals with poor condition. None of the blood parameters measured was related significantly to either season or cub survival.