The monthly rainfall at five localities measured over the previous 21 years, and the average monthly run-off of two rivers observed during the past ten years were studied using time-series analysis. Localities and rivers are situaded in a karstic basin (basin of the Valserine) of the French Jura Mountains. Examination of the different spectra showed that at least three periods contribute to the variance of these series: 24 months, 12 months and 4 months. A climatic and hydrogeological explanation is given for these three components. As the band-width was not always narrow enough to distinguish the annual peak from the biennial, a moving-average filter was used to resolve these two components. A good coherency was observed within rainfall series and within run-off series but the coherency between rainfall and run-off was not significant for periods of 12 and 4 months. The 12-and-4-month components are simple harmonic processes whilst the biennial component is a second-order autoregressive process.