This study sought to determine if the generalization gradient obtained along the frequency dimension of acoustic clicks would be similar for a group of rabbits simultaneously conditioned to stimuli consisting of 15 pulses of brain stimulation per second and 25 clicks per second, and a group of rabbits conditioned to stimuli consisting of 15 and 25 acoustic clicks per second. A summated generalization gradient along the frequency dimension of acoustic clicks was obtained from rabbits conditioned to ESB and acoustic clicks if the electrode tips were located in the medial geniculate body. If the electrode tips were located in nonauditory brain structures (e.g., optic tract, lateral geniculate body, midbrain reticular formation), the generalization gradient obtained along the frequency dimension of acoustic clicks resembled one obtained from a group of rabbits initially conditioned to a stimulus consisting of 25 acoustic clicks per second. The results are discussed in terms of providing evidence for a frequency theory of learning and questioning the usefulness of a "labeled lines" code for carrying information within the central nervous system.