We have analyzed the interplay between an externally added noise and the intrinsic noise of systems that relax fast towards a stationary state, and found that increasing the intensity of the external noise can reduce the total noise of the system. We have established a general criterion for the appearance of this phenomenon and discussed two examples in detail. DOI: 10.1103/PhysRevLett.86.950 PACS numbers: 05.40.Ca, 05.40. -a For a long time, noise was considered to be only a source of disorder, a nuisance to be avoided. Recently, this view has been changing due to several phenomena that show constructive facets of noise. Among them, the most widely studied is the phenomenon of stochastic resonance, where the addition of noise to a system enhances its response to a periodic force [1,2]. This counterintuitive aspect of noise has been found under a wide variety of situations, including bistable [3] and monostable [4] systems, nondynamical elements with [5] and without [6] threshold, and pattern forming systems [7]. Similar constructive outcomes are also found in other remarkable phenomena, such as noise induced transitions [8] and noise induced transport [9]. To some extent, the presence of noise is an unavoidable feature, and as one moves from macroscopic to microscopic scales that presence becomes more and more prominent. To withdraw the noise, it is customary to reduce as much as possible all of the external noise sources that affect the system since it still seems paradoxical that adding noise might result in a less noisy system.In this Letter we show that the intrinsic noise displayed by some systems can substantially be reduced through its nonlinear interplay with externally added noise and we establish sufficient conditions for this phenomenon to occur. The systems we consider are those relaxing fast to a stationary state where their properties are completely determined by some parameters: the state of the system, denoted by I͑t, V͒, is a function of some input parameters, denoted by the set V. These systems are usually called nondynamical systems. For the sake of simplicity, we consider the case of a single input parameter, i.e., V ϵ V . The temporal dependence of the state of the system takes into account the intrinsic fluctuations. This stochastic behavior can be described by the mean value ͗I͑t, V ͒͘ H͑V ͒and the correlation functionwhere ͗.͘ indicates average over the noise. For practical purposes, it is convenient to write I͑V , t͒ aswhere j͑t, V ͒, with ͗j͑t, V ͒j͑t 1 t, V ͒͘ G ͑V , t͒, represents the intrinsic noise. When the characteristic time scales of the fluctuations are smaller than any other entering the system,G͑V , t͒ G͑V ͒d͑t͒, and the power spectrum of the fluctuations is flat for the range of frequencies of interest. The function H͑V ͒ corresponds to the deterministic response and G͑V ͒ corresponds to the intensity of the intrinsic fluctuations.We now analyze how adding external noise affects the output of the system. We consider that a random quantity z ͑t͒ is added to a constant in...