An oceanographic survey of the eastern tropical Pacific Ocean in August–November 1990 found a productive, nutrient-rich, moderately high-chlorophyll surface layer in two oceanic upwelling regions: the equatorial divergence, especially east of the Galapagos, and the countercurrent divergence out to 105°W, > 1,000 km west of the Costa Rica Dome. Although NO3 is not depleted in upwelling regions, relationships among nutrient concentrations and temperature in 1986–1988 data from the same area show that NO, is the the first macronutrient to be depleted in adjacent, less-productive regions. A three-dimensional, two-layer box model of NO3 flux within and into the euphotic zone gives estimated rates of new production that are ∼29% of measured rates of 14C phytoplankton production. Persistence of excess (saturating) NO3 in the euphotic zone exceeds 1 yr under high-nutrient, low-chlorophyll conditions off the equator where weak upwelling, or down-welling, occurs. These results indicate substantial control or limitation of NO3 utilization and productivity in nutrient-rich oceanic regions of the eastern tropical Pacific.