Rates of new production in the warm waters of the tropical Pacific and their seasonal to interannual variability are estimated by measurement of 15N-nitrate uptake along the equator in the western and central (145 ¿E-165 ¿W) equatorial Pacific. Measurements were carried out during non-El Ni¿o conditions and two examples of anomalous conditions: the moderate El Ni¿o in 1994-1995 and the strong El Ni¿o in 1997-1998. Variations in new production are explained in relation to changes in physical, chemical, and biological environments during the El Ni¿o-Southern Oscillation cycle. During non-El Ni¿o conditions in the western region the rates of integrated new production over the euphotic zone were low (0.15 mmol m-2 d-1) and likely limited by availability of nitrate. In the central region, new production was higher (1.37 mmol m-2 d-1) owing to upwelling of nitrate-rich subsurface waters. The east-west asymmetry of observed variables in the western and central equatorial Pacific is altered during El Ni¿o conditions owing to the eastward expansion of the western Pacific Warm Pool. In the western region, during the moderate El Ni¿o in 1994-1995, the integrated new production increased slightly compared to non-El Ni¿o conditions, while during the strong El Ni¿o in 1997-1998, it was higher by a factor of 10 than in non-El Ni¿o conditions. In the central region during the 1994-1995 El Ni¿o, the rates of new production markedly decreased compared to non-El Ni¿o conditions. During the El Ni¿o in 1997-1998, the integrated rates of new production were comparable to non-El Ni¿o conditions, but the vertical distribution showed a downward displacement of maximum rates. The results of our study suggest that interannual variations in new production in the western and central equatorial Pacific correlate well with the change of the nitracline depth during the eastward expansion of the Warm Pool and depend strongly upon the severity of the El Ni¿o event. ¿ 2001 American Geophysical Union