A variety of methods for subject selection and test procedures have been used for the determination of normal values and reference equations for maximal inspiratory pressure (MIP). In the cross-sectional study described here, we made MIP measurements on 668 men and women in the Baltimore Longitudinal Study of Aging (BLSA), using a standardized electronic procedure. Results were combined with spirometric and anthropometric measurements. After subjecting them to rigorous health screening, we analyzed a well-defined, healthy subgroup of 139 men and 128 women with a wide age range (20 to 90 yr), using multiple linear regression, for the purpose of determining the effect of age, other correlates, normal values, and gender-specific reference equations for MIP. The gender effect was strong, with the average MIP values of the men being about 30% higher than those of the women (101 cm H2O and 72 cm H2O, respectively). The reference equation for men is: MIP +/- standard error of the estimate (SEE) = 126 - 1.028 x age + 0.343 x weight (kg) +/- (22.4); and for women: MIP +/- SEE = 171 - 0.694 x age + 0. 861 x weight (kg) - 0.743 x height (cm) +/- (18.5). These equations may be used for the assessment of inspiratory muscle strength.