Excitable media, which range from autocatalytic chemical systems(1-3) to biological cells and tissues(4-7), can maintain organized structures in the form of rotating spiral waves of excitation(8-11). The dynamics of spiral waves in two-dimensional systems have been shown to be susceptible to control by external fields (such as electric, thermal and optical)(12-14). In three dimensions, the analogues of spiral waves are scroll waves(9,15,16), Here we show that an external held-a temperature gradient-can be used to control a particular class of scroll waves called scroll rings. The gradient allows scroll rings to be precisely oriented in space, and their spontaneous shrinkage to be accelerated, decelerated or even reversed (so that the ring expands). The temperature gradient also influences the lifetimes of the scroll rings. We suggest that these dynamics are likely to be generic to other types of field gradients and other excitable media.