We examined responses to pure tones and exponentially frequency-modulated (FM) stimuli in the inferior colliculus of ketamine anesthetized rats. All units responded to both pure-tone and FM stimulation: units responding selectively to FM stimuli were not found. The comparison between responses to many different FM sweeps revealed that activity was elicited when the instantaneous frequency of a FM sweep entered the unit's pure-tone tuning curve. Units were tuned to the rate of frequency modulation. Most modulation rate transfer functions had bandpass characteristics. Best modulation rates covered a range from 4.8 to 1904 octaves/s with more than 90% between 10 and 400 octaves/s. In contrast to previous studies, modulation direction was not coded in unit responses and only few units demonstrated a weak change in response strength when sweep direction was altered. This is at least partly attributable to the FM stimulus design which, in the present study, was adapted to the logarithmic frequency representation in the rat auditory system and carefully matched to the units' pure-tone responses area. In spite of the close relationship between pure tone and FM response behavior, modulation rate tuning cannot be completely explained on the basis of the units' pure-tone responses.