Using a method introduced by Harris  AP unmasking was investigated in normal guinea pigs. AP unmasking existed in every guinea pig investigated and proved to be stable over measuring periods as long as 7 h. The average standard deviation of unmasking magnitude was 12%. AP unmasking strength was defined as the average unmasking magnitude across across suppressor levels from 0 to 100 dB SPL at constant masker and test-tone level and constant masker, test-tone and suppressor frequency. The relation between AP unmasking strength and AP thresholds was investigated. AP unmasking strength decreases with increasing AP threshold at the suppressor frequency. No relation with AP thresholds at other frequencies was found. AP unmasking areas were determined along with the corresponding AP tuning curves. High-frequency unmasking was found to be more prominent and more stable than low-frequency unmasking in guinea pig. From a comparison with other studies on AP unmasking and single fibre two-tone suppression it was concluded that a species difference exists with regard to the presence of low-frequency AP unmasking and low-frequency single fibre two-tone suppression.