Two organic broilers rearing systems, each associated with a broiler house of 75 m2 and an outdoor run of 2500 m2, were studied. In the first system, the outdoor run consisted of grassland (GRASS) but in the second, the outdoor run was planted with oaks (TREE). Two batches of broilers were studied from winter to spring (WS) and from summer to autumn (SA). Indoor and outdoor airborne concentrations of NH3, N2O, CH4, CO2 and H2O were measured using a photoacoustic infrared analyser. Air flow rate through the broiler house was estimated using SF6 tracer gas. The final amount and composition of the manure was determined. A mass balance was determined for N, P and water. Results showed that in SA, more nutrients were excreted in the outdoor run (41%) compared with WS (11%). During the WS season, outdoor excretion was greater in the TREE system but this was not observed during SA. Ammonia indoor emissions were higher during WS (200-283 mg [NH3] d-1 bird-1) than during SA (99-116 mg [NH3] d-1 bird-1). Total emissions from the systems (indoor þ outdoor) were assessed using our values and data from the literature for outdoor emissions. This confirmed that total NH3 emissions were in the same range as emissions of broilers reared in closed houses. N2O emissions were higher during WS but represented less than 0.5% of indoor N excretion. Total CH4 emissions were very low (40-60 mg [CH4] d-1 bird-1), particularly in comparison with emissions in other species.