We study dynamics of rational maps of degree at least 2 with coefficients in the field ${\open C}_{p}$, where p>1 is a fixed prime number. The main ingredient is to consider the action of rational maps in p-adic hyperbolic space, denoted ${\open H}_{p}$. Hyperbolic space ${\open H}_{p}$ is provided with a natural distance, for which it is connected and one dimensional (an ${\open R}$-tree). These advantages with respect to ${\open C}_{p}$ give new insight into dynamics. In this paper we prove the following results about periodic points; we give applications to the Fatou/Julia theory over ${\open C}_{p}$ and to ultrametric analysis in forthcoming papers. We prove that the existence of at least two nonrepelling periodic points implies the existence of infinitely many of them. This is in contrast with the complex setting where a rational map can have at most finitely many nonrepelling periodic points. On the other hand we prove that every rational map has a repelling fixed point, either in the projective line or in hyperbolic space. So the topological expansion of a rational map is detected by some fixed point.