Few phenomena are completely described by a single number. For example the size of a population of rabbits can be represented using one number but to know its rate of change we should consider other quantities such as the size of predator populations and the avaliability of food. In this paper we discuss models the evolution of the two populations in a predator-prey system of differential equations where one species eats another foxes (F) eats rabbits (R). Our model has two dependent variables that are both functions of time. A solution of the system can be graph so we get informations about behavior of them.