Pistachio yields normally alternate between high and low values for consecutive years due to management and genetic effects. In this study, irrigated and non-irrigated treatments of old pistachio trees (Pistacia vera L.) were compared for 5 years from 1979 to 1983. Two irrigation treatments consisting of irrigation scheduling at 20- and 30-day interval were compared with the control treatment of no irrigation. Results showed that irrigation practice in pistachio orchards not only increased fruit yield, but also suppressed periodicity, the low fruit yield in alternating years. The frequent irrigation of 20-day interval nearly doubled the dry fruit yield irrespective of whether it was the off-yielding year, compared to non-irrigated control treatment. Three-years average for the irrigation at 20 days interval was 5710 kg per ha during the on-yielding years (1979, 1981 and 1983); whereas, the control treatment of no irrigation yielded only 3450 kg per ha during the same period. Two-year average of the non-irrigated treatment, during the off-yielding years (1980 and 1982) was 1730 kg per ha; whereas, the irrigated treatment of 20-day interval, for the same period, showed significantly (P less-than-or-equal-to 0.05) higher fruit yield (4030 kg per ha). Irrigation treatments, during both on and off-yielding years, had no effect on fruit quality characteristics such as nut weight, blank and split shell percent, etc. The root activity under frequent irrigation treatment was confined to shallower soil depth than the longer interval irrigation treatment.