2f1-f2 otoacoustic emissions have been recorded from the rat cochlea during its development. Acoustic responses were recorded at 3, 5 and 7 kHz using a fixed value of the f2/f1 ratio (= 1.17). The first 2f1-f2 acoustic responses were obtained at 12 days after birth for 2f1-f2 = 7 and 5 kHz, and 2 days later for 2f1-f2 = 3 kHz. Adult-like patterns of the acoustic responses were achieved by day 18 for 2f1-f2 = 3 kHz, by day 20 for 2f1-f2 = 5 kHz and by day 28 for 2f1-f2 = 7 kHz. These results are discussed in relation to the available anatomical and functional data on the cochlear development of the rat. The delayed appearance of the 3 kHz acoustic responses might be related to the basal-to-apical gradient of morphological cochlear maturation. The fact that the 2f1-f2 otoacoustic emissions reached adult characteristics from the low to high frequencies is consistent with the development of the tuning properties of the basilar membrane. The long development of the 2f1-f2 acoustic responses at 7 kHz suggests that the organ of Corti undergoes subtle changes well after the end of its apparent maturation.