An apparatus for measuring the flexural rigidity in the longitudinal direction of flat items of mail transported one after another on a conveying path includes a control device, a sensor for detecting the edges of the items of mail and signaling the control device when a mail item reaches a selected bearing point, a deflection apparatus that moves into the conveying path under the control of the control device to bend individual mail items and means for determining one or both of the magnitude of the deflection of the items of mail and the magnitude of the deflection force, the control device progressing signals from one or both of the sensor and measuring means to determine the flexural rigidity of the mail item.