Historical air quality and emissions trend data for the Los Angeles region were used to check the EKMA isopleth method of relating ozone concentration changes to precursor emission changes. Trends in ozone and ozone precursors (NMHC and NOx) were estimated from data for the period 1964 to 1978. Emission trend calculations show that basinwide hydrocarbon emissions decreased continually, with a net reduction of 29% over the study period. Estimated basinwide NOx emissions rose rapidly from the middle 1960s to the early 1970s and then levelled off; the net increase over the study period was 34%. Sensitivity analysis reveal that predicted ozone trends are moderately sensitive to the specific EKMA simulation conditions. Predicted ozone trends are extremely sensitive to the NMHC/NOx ratio. The EKMA model performs fairly well in the validation studies. There is, however, a general tendency for predicted ozone trends to underestimate historical decreases in actual ozone trends. One reason for the discrepancy appears to be error in the choice of an NMHC/NOx ratio.