Few quantitative data are available on the ways in which venomous snakes respond to humans. We took advantage of extraordinarily high numbers of endemic pit vipers on the small island of Shedao in northeastern China, to quantify snake responses. We approached free-ranging pit vipers in their ambush sites (either in trees or on the ground) and recorded their behavior. The snakes' responses to our approach depended upon (1) the intensity of the stimulus; (2) attributes of the snake (size class, body temperature, whether it had fed recently); and (3) the snake's location (in an arboreal or terrestrial site). Most snakes tolerated close approach. Juvenile pit vipers struck more often than adults, and warmer snakes were more likely to flee, display, or strike rather than rely on crypsis. Snakes on the ground were more likely to flee or strike than were those in trees. Many of the same patterns were seen in our laboratory trials. For example, striking was more frequent in juveniles than adults, in hotter snakes, and in snakes that displayed (tail-vibrated). Both strike speeds and tail-vibration rates increased with body temperature. Responses of Shedao pit vipers to human approach differ from those reported for other snake species.