This study examined the impact on groundwater quality of conventional and slow‐release N fertilizer to small, grazed watersheds in eastern Ohio. Three small watersheds (each less than 1 ha) received 56 kg N/ha annually as NH4NO3 for 5 yr. For the next 10 yr, one watershed received 168 kg N/ha annually as NH4NO3 and two others received the same amount of N as methylene urea, a slow‐release fertilizer. Shallow groundwater samples were collected from springs and analyzed. After the 5‐yr prestudy period, NO3‐N levels in the groundwater from the three watersheds were in a 3 to 5 mg/L range. Groundwater NO3‐N concentrations increased slightly during the first 3 yr at the higher N fertilizer rate, though they remained in the 3 to 5 mg/L range. Nitrate‐N levels increased more sharply during the rest of the study. Although these NO3‐N levels varied more between the growing and dormant seasons than when lower rates of fertilizer were applied, they eventually reached a slower rate of increase. During the 9th and 10th yr of the high application, seasonal NO3‐N levels in groundwater ranged from 10 to 16 and 7 to 14 mg/L from the watersheds receiving NH4NO3 and methylene urea, respectively. This study showed that 168 kg N/ha was too much for this system, regardless of whether conventional or slow‐release N was used.