The effect of the microbially produced herbicide bialaphos on ammonium accumulation and photosynthesis of white spruce (Piceaglauca(Moench) Voss) and red pine (PinusresinosaAit.) seedlings was studied under controlled environmental conditions. Ammonium accumulation increased markedly during the first 48 h for seedlings treated in June and was greater in red pine than in white spruce. Ammonium accumulation also increased with higher doses of bialaphos. In contrast, the accumulation of ammonium following bialaphos application in August was minimal for both species and for doses up to an equivalent of 3.0 kg active ingredient (bialaphos) per hectare. Continuous measurements of photosynthesis for the first 12 h following bialaphos application showed a slight negative effect of the herbicide on photosynthetic activity within 2 h of treatment. The decrease was greater for red pine than for white spruce. Photosynthesis measurements taken under three different levels of photosynthetic photon flux density 24 h after bialaphos application showed a greater decline with increasing photosynthetic photon flux density and with higher doses of bialaphos. Furthermore, the decline was greater in red pine than it was in white spruce. When photosynthesis was followed over a 7-day period, a large decrease was observed during the first 24 h, after which time photosynthesis stayed relatively constant. The decrease was greater with higher doses and was greater for red pine than for white spruce. There was a strong negative correlation between photosynthetic rate at day 7 and both the dose of bialaphos and the development of chlorotic foliage. These results help provide a physiological basis for understanding previously reported morphological and growth responses.