Twelve highly trained listeners detected tonal signals in the absence of external masking noise. The signals were 150 msec long and ranged from 125 to 4000 Hz. Fitted psychometric functions were steeper at high than at low frequencies requiring between 3‐ and 8‐dB increase in signal level to raise performance from 60% to 95% correct. The signal levels that yield 76% correct detection in a two‐alternative, forced‐choice psychophysical procedure were in good agreement with the International Standards Organization's recommended values of audiometric zero. The results are consistent with a limitation imposed on auditory sensitivity by a “system noise” which is composed of the noise in the ear canal at low frequencies, and a low‐level (−11 dB SPL) component of unspecified origin at medium and high frequencies. No qualitative differences were observed between detection without external masking noise and previous data on the detection of tones masked by noise.