The net nitric oxide (NO) flux from the soil of a tropical rain forest site in Reserva Biologica Jaru (Rond¿nia, Brazil) was continuously measured in May and during September--November 1999 using dynamic chambers. The median net NO flux was 4.7 ng N m-2 s-1 in May (1000 individual measurements) and 4.0 ng N m-2 s-1 in September--November (4200 individual measurements). The daily variation of the net NO flux was positively correlated to the daily variations of soil temperature. Minor rain events (<10 mm) had no significant influence on the 24-hour mean net NO flux. Following larger amounts of rain, a 40% reduction of the 24-hour mean net NO flux up to a 150% increase was observed. During the September--November experiment the soil NO concentration and the soil NO bulk diffusion coefficient were measured to calculate the net NO flux using an indirect method. The soil NO concentration was continuously measured at 0.03-m depth. It ranged from 20 to 460 ppb and was positively correlated with soil moisture and soil temperature. The soil bulk diffusion coefficient determined from 222Rn flux and soil-air activity gradient measurements ranged from 2.7 ¿ 10-7 to 9.1 ¿ 10-7 m2 s-1 and was parameterized with soil air-filled pore space. The median net NO flux calculated by the indirect method was 3.9 ng N m-2 s-1 (1600 individual measurements).