Soil microorganisms and the processes they control are essential for long-term support of agricultural systems; therefore the effects of pesticides on microbial biomass of the soil and their activity have attracted many attentions. Although widespread use of chemical compounds in agriculture increase the amount of crop, it leads to soil pollution and surface water as well as ground water pollution which are a threat to human health. For this reason investigation and control of the process of their degradation and their side effects in agricultural soils are of great importance. One of the herbicides which is largely used in Iran and other parts of the world is Atrazine. Atrazine is a major herbicide for controlling wide leaf weeds in corn and other graminaceous fields. The objectives of the current study were to investigate: 1- the fate of Atrazine in soil following incubation under optimum conditions and 2- the effects of Atrazine on soil net N mineralization, arginine ammonification, respiration rate, metabolic quotient and soil amidohydrolases. Soil samples were collected from 0-15 cm layer of agricultural soils from Shervedan and Juzdan ( Isfahan , Iran ). To achieve the two goals of study, two independent experiments were run. First, Different concentrations of Atrazine (0, 0.2, 0.4, 0.6, 1, 10, 50, 100, 250, 500 and 1000 mg atrazine kg -1 dry wight soil) were added to soils. Triplications of soils were incubated for 3 and 60 days. Aerobic N mineralization, arginin amonification, urease, L-glutaminase and L-asparaginase activities were measured. Second, the release of ammonia was monitored in a long incubation study in order to recognize the kinetic behavior of Atrazine decomposition in calcareous soils. Results indicated that in both soils of Shervedan and Juzdan, aerobic N mineralization and the activity of urease and L-glutaminase enzymes as time elapsed significantly increased. The mentioned biological properties were positively responded to enhancement of Atrazine concentration. As time elapsed, arginin amonification decreased. The activity of L-asparaginase was irresponsive to Atrazine addition as well as incubation time. Apparently L-asparaginase did not follow the trends observed for the activity of soil urease and L-glutaminase. Among the soil biological properties studied in the present research, metabolic quotient was the most responsive one to addition of Atrazine. It has already been extensively reported that the metabolic quotient is a sensitive index to stress conditions. Actually, the metabolic quotient indicate the cell maintenance costs and it is likely that Atrazine has generally increased the stresses for soil microbial populations so that