Using data from two studies that elicited nitrate health risk and exposure perceptions before and after a well testing program, this paper investigates whether participants update their risk perceptions with new infonnation. Graphical analyses demonstrate that, in the aggregate, updating occurs when well test infonnation is provided. In particular, uncertainty about safety and exposure appears to be substantially reduced, and perceptions correspond to the distribution of nitrates. Statistical analyses indicate that individual updating of perceptions is a systematic function of prior perceptions and nitrate test levels. Evidence that updating occurs demonstrates that public infonnation programs can be effective in modifying risk perceptions, and offer a critical first step in assessing the cost-effectiveness of such programs