As the Thai economy grew rapidly after 1985, agriculture became more intensive through the increasing use of chemical fertilizer and mechanization. This study aimed to analyze the nitrogen (N) cycle related to agricultural activities in Khon Kaen Province in Thailand during 1990–1992 and 2000–2002, and on the changes in utilization of local organic resources and the N load to the environment. A model of the N cycle was constructed including compartments for farmland, crop yield, crop residue, food factory, livestock, humans, market, hydrosphere and atmosphere. N flows among the compartments in the model were estimated from data derived from Thai agricultural statistics, related reports and journal articles, interviews with farmers and food factory staff, field observation and information from Thai experts. N flow through livestock declined because of a decrease in the number of buffalo raised, which reduced the production of animal manure. N returned to farmland in crop residues increased because sugarcane cultivation, and crop residues, increased and the burning of rice straw decreased. An increase in chemical fertilizer application increased N input to farmland for crop production. N balance in farmland changed from -27 kg ha-1 year-1 in 1990–1992 to +6 kg ha-1 year-1 in 2000–2002, which improved soil N depletion. Because N leaching and erosion from farmland were low, water pollution in farmland is expected to be low. Human waste was not used or treated, and water pollution from human waste would be expected in housing areas. Analysis of indices of the N cycle showed that the stock of soil N in farmland supported agricultural production in 1990–1992, and that N inflow from outside the area (chemical fertilizer) supported agricultural production in 2000–2002. However, efficiency of N use for agricultural production did not improve.