A one-dimensional photochemical model is used to calculate the rate at which ammonia would have been destroyed in the earth's atmosphere for assumed NH3 mixing ratios of 10-8 to 10-4. The calculated NH3 destruction rates are then compared with possible abiotic and biotic sources for ammonia. Abiotic sources were probably sufficient to maintain the NH3 mixing ratio of 10-5 or more necessary to produce significant greenhouse warning. The atmospheric ammonia concentration may have increased during the Archean as a result of biological activities but probably remained below the level required to generate measurable thermal effects.