Fifty-two new K-Ar dates for Upper Cenozoic volcanic rocks from north Chile and southwest Bolivia are presented, together with a compilation of previously available dates from this region. These dates are combined with calculations of volumes of lava and ignimbrite for a segment of the volcanic province (19°30'S to 22°30'S) to identify fluctuations in the level of volcanic activity during the last 24 Ma. Histograms of volumes against time have been plotted for each half-degree quadrant. In the southern half of the study area, there were peaks of activity in the periods 12 to 9 Ma and 6 to 3 Ma. In the northern half, a large proportion of the material was erupted in the period 6 to 0 Ma. This regional variation suggests that localized factors may govern the rate of volcanic output and complicates attempts at correlation with "pulses" of volcanic activity recognized elsewhere in the Pacific region. There is no conclusive evidence for volcanic episodes synchronous over such wide areas. A simple correlation between changes in spreading behaviour and changes in levels of volcanic activity is unlikely, in view of the complexity of the interactions at destructive plate margins. The rate of continental crust accretion from volcanic processes must be much less than that due to intrusive processes to account for the thickening of the Andean crust since the Jurassic.