Four environmental monitoring stations are operated at locations on the perimeter of Brookhaven National Laboratory, about 2.5 km distant from the Brookhaven Graphite Research Reactor (BGRR) stack. During 1964 high volume samplers containing particulate and charcoal filters in sequence were operated at these stations. The BGRR cooling-air effluent contains about 8 mc/day of 131I in a concentration of 7 × 10−10μc/cc, apparently originating from fuel element surface contamination of 1 μg/ft2. Using improved techniques of gamma spectroscopy, annual average concentrations between 4 × 10−15μc/cc and 1 × 10−16μc/cc were measured at the perimeter stations. These ranged between 200 and 15 per cent of those calculated for these locations by the BNL Meteorology Group. A concentration of 131I of 7.2 × 10−13μc/cc was measured on 28–29 October 1964, in a single day peak of air activity attributable to the Chinese weapons test of 16 October. Grass concentrations of between 1.0 and 3.0 × 10−7μc/g were found during the next 2 weeks. An 131I deposition velocity of 0.25 ± 0.10 cm/sec was calculated from the initial deposition. A concentration of 32 pc/l, was found on 1 November in a nearby milk supply. This concentration decreased over the next few weeks with a 4.8 day “effective” half life. From 12–16 November about 300 mc of 131I vapor was inadvertently released by the Hot Laboratory from the BGRR stack. The measured integrated air concentrations at the three stations downwind for some portion of the release ranged from twice to one-half of those predicted for short-duration releases.