We report the influence of equivalence ratio and air preheat temperature on nitric oxide (NO) concentration in high-pressure heptane spray flames. The burner used was based on the lean direct-injection design and incorporated a pressure-atomized, hollow-cone spray nozzle. Helical vanes in the air passage coupled with a divergent exit and preheated air produced a strongly swirling, clean blue flame. NO concentration profiles were measured at 4.27 atm using laser-induced fluorescence (LIF) at five axial heights above the burner. The uniformity of NO throughout the central region of the flame demonstrated the well-mixed nature of the recirculation zone. Measurements were taken at the 40 mm centerline height to determine the effects of primary equivalence ratio ( p ס 1.0 to 0.8) and air preheat temperature (T air ס 375 K to 575 K). The results strongly suggest that NO formation occurs in near stoichiometric regions of the flame and is subsequently diluted with excess air. A residence time effect is evident and apparently scales as the mass flow rate of air relative to that for stoichiometric combustion, yielding a scaling of 2 p the NO (ppm). Moreover, moderate increases in preheat temperature produce significant increases in NO (ppm) levels, suggesting thermal NO production.