The invention relates to a method for ascertaining a resistance value (Z) between a first contact (2) and a second contact (3) in a subscriber line interface circuit (4), where a protective circuit (9) for protecting the subscriber line interface circuit (4) against overvoltages is provided between the two contacts (2, 3) and comprises a parallel circuit containing a protective capacitor (5) with two resistors (7, 8) connected in series via a node (K), the node (K) being connected to a third contact (10) in the subscriber line interface circuit (4), where the method has the following steps: a predetermined charging voltage (U.sub.Charge) is applied to the protective capacitor (5); a threshold voltage (U.sub.TH) is calculated on the basis of the resistance values (R.sub.1, R.sub.2) of the two resistors (7, 8) and the applied charging voltage (U.sub.Charge); a measured voltage (U.sub.M) tapped off across one of the two resistors (7, 8) is measured while the protective capacitor (5) is discharging; the measured voltage (U.sub.M) is compared with the calculated threshold voltage (U.sub.TH); a period (.DELTA.t) between the start of the discharging of the protective capacitor (5) and the time at which the measured voltage (U.sub.M) is the same as the threshold voltage (U.sub.TH) is ascertained; and the resistance value (Z) is calculated using the ascertained period (.DELTA.t) and the resistance values (R.sub.1, R.sub.2) of the two resistors (7, 8).