Exposure of pea (Pisum sativum L. cv Sparkle) roots to light suppressed nodulation and induced an increase in ethylene production by roots. Dim light did not affect the number of infections per centimeter of root on the primary root, but most infections were blocked when the infection thread was in the epidermis or in the outer cortex. This is the same stage of infection on lateral roots that is blocked by exogenous ethylene. Silver, an inhibitor of ethylene action, increased nodule number on roots exposed to dim light. Exposure of pea roots to nitrate also suppressed nodulation and induced increased ethylene production by roots. However, induction of ethylene production from roots by nitrate was less than that induced by dim light. Nitrate decreased the number of infections per centimeter of lateral root. With 25 mm nitrate, about half of the infections that occurred were blocked in the epidermis or in the outer cortex. Silver did not reverse the inhibitory effects of nitrate on nodulation. Our data indicate that the inhibitory effect of light may occur via increased ethylene production, but they do not support the hypothesis that ethylene mediates nitrate control of nodule number. The possible role of ethylene in regulating nodule number should be studied in experiments in which light is excluded from the roots.