The present experiments were carried out in order to evaluate the influence of head injury on lung compliance as determined by in vivo methods. Lethal or ‘sub-lethal’ head injury in monkeys did not alter gross lung appearance but caused a 36-percent in vivo compliance decrease. This compliance decrease was not ameliorated with isoproterenol which tends to eliminate airway smooth muscle involvement. Systemic hemodynamic causal factors were reasonably ruled out in at least three animals by maintenance of constant aortic and central vena cavai pressures.