For time series exhibiting strong periodicities, standard (linear) surrogate methods are not useful. We describe a new algorithm that can test against the null hypothesis of a periodic orbit with uncorrelated noise. We demonstrate the application of this method to artificial data and experimental time series, including human electrocardiogram recordings during sinus rhythm and ventricular tachycardia. DOI: 10.1103/PhysRevLett.87.188101 PACS numbers: 05.45.Tp, 05.10. -a, 87.19.Nn The method of surrogate data [1] is widely applied to test the null hypothesis that an observed time series is a typical realization of the output of a specific class of dynamical systems. This method is widely used in the analysis of experimental time series and provides a powerful tool in the search for determinism in apparently stochastic data. However, the current surrogate techniques have very limited utility when applied to a time series with a strong pseudoperiodic behavior.The surrogate algorithm we describe in this Letter generates pseudoperiodic surrogates (PPS). This method is based on the well-known local-linear modeling methods described by Mees [2] and Sugihara and May [3]. Previously, Small and Judd advocated [4] and implemented [5] nonlinear radial basis modeling routines [6,7] as a form of surrogate hypothesis testing. The method we describe here is simpler and tests a more specific null hypothesis. This method may be applied to test against the null hypothesis of a periodic orbit with uncorrelated noise in the very large number of experimental systems that exhibit pseudoperiodic behavior.By contrast, the three most successful, and widely applied, algorithms test for membership of the class of (i) independent and identical distributed (IID) noise processes, (ii) linearly filtered noise processes, and (iii) static monotonic nonlinear transformation of linearly filtered noise processes [1,8]. For time series data exhibiting strong pseudoperiodic behavior, the null hypotheses of IID or colored noise are obviously false. Therefore, apart from serving as a "sanity check," these existing algorithms are of limited use for such data.For a pseudoperiodic time series, Theiler and Rapp suggested an alternative algorithm [9]: cycle shuffled surrogates. Analogously to IID noise surrogates, cycle shuffled surrogates are produced by shuffling the individual cycles within a time series. Hence, intracycle dynamics are preserved but intercycle dynamics are not. However, even with this new approach Theiler noted spurious long term correlations in the autocorrelation plot [9] for cycle shuffled surrogates. Furthermore, if the peak or troughs do not occur at precisely the same values, surrogates generated by this method are not able to preserve both stationarity and continuity. By shifting individual cycles vertically, individual cycles can be matched and continuity will be preserved. However, such a transformation introduces nonstationarity in the surrogates that is absent in the original.Each of these techniques is commonly applied to...