The fluctuations in amplitude and phase characteristic of narrow bands of noise are discussed in relation to the difference limen for the center frequency of such bands of noise. Two models are elaborated: a temporal model on which pitch is derived from the time pattern of nerve impulses, and a “place” model in which pitch is derived from the patterns of excitation on the basilar membrane. These models predict how the difference limen for center frequency will vary as a function of the bandwidth of the noise. The temporal model predicts certain effects for very narrow bands of noise which would not be expected on the “place” model. The predictions from the two models were tested using bands of noise synthesized by the addition of a large number of sinusoids. These bands of noise had an essentially rectangular spectrum. The results conform with the predictions of the temporal model for center frequencies of 2 and 4 kHz, and with the predictions of the “place” model for a center frequency of 6 kHz. It is concluded that the pitch of pure tones and very narrow bands of noise is determined primarily by a temporal mechanism for frequencies up to 4 kHz, and that at some frequency above this a “place” mechanism becomes predominant.