A new model of Genetic Programming with variable size population is presented in this paper and applied to the reconstruction of target functions in dynamic environments (i.e. problems where target functions change with time). The suitability of this model is tested on a set of benchmarks based on some well known symbolic regression problems. Experimental results confirm that our variable size population model finds solutions of the same quality as the ones found by standard Genetic Programming, but with a smaller amount of computational effort. Categories and Subject Descriptors I.2.2 [Artificial Intelligence]: Automatic Programming General Terms Algorithms, Performance Keywords Genetic Programming, Variable Size Population, Dynamic Optimization