Spectrum analysis of 32 tree-ring chronologies from Argentina and Chile yields evidence for two peaks with periods 19.2±1.6 years (30 out of 32 records) and 10.5±0.4 years in 22 instances. Tests by thet-statistic show that the long-period peak is significant at a confidence level of 99%. This signal is identified as the luni-solar 18.6-year M _n term reported earlier byCurrie (1983) in two treering chronologies from the same region, and later in tree-rings from North America, Tasmania, New Zealand, and South Africa (Currie, 1991a-c). Amplitude and phase of the M _n signal are nonstationary with respect to both time and geography. In particular, abrupt 180° phase changes in wave polarity are often observed.