1. Several recent studies of the relation between blood pressure and ageing have been re-examined, and it is concluded that none satisfactorily investigates the possibility of separate influences of age and attained pressure in determining the rate of change of pressure with time.
2. If the rate of increase of pressure is proportional to the attained pressure, pressure is an exponential function of age. An attempt has been made to determine the relation of blood pressure with age by fitting curves to individuals' measurements made over periods of 15 1/2 and 17 1/2 years in two epidemiological surveys in South Wales.
3. Three models were used: (1) a linear regression, (2) an exponential function, and (3) a model based on the hypothesis that an individual's pressure fluctuates about a constant mean until a variable age at which it increases at a constant rate. Neither model (2) nor model (3) was significantly better than model (1), but this may have been largely due to the small number of measurements (four) for each subject.
4. A positive slope to the linear regression of pressure on time was obtained for most adults. In middle-aged subjects the distributions of these regression coefficients appear unimodal. The rate of increase was higher in those with initially raised values but increased with age independently of the blood pressure level. In these populations some factor related to ageing appears to play a rôle in causing this increase in pressure in addition to any possible direct influence of the attained pressure.