  The modelling and analysis of biological systems has deep roots in
Mathematics, specifically in the field of ordinary differential equations
(ODEs). Alternative approaches based on formal calculi, often derived from
process algebras or term rewriting systems, provide a quite complementary way
to analyze the behaviour of biological systems. These calculi allow to cope in
a natural way with notions like compartments and membranes, which are not easy
(sometimes impossible) to handle with purely numerical approaches, and are
often based on stochastic simulation methods. Recently, it has also become
evident that stochastic effects in regulatory networks play a crucial role in
the analysis of such systems. Actually, in many situations it is necessary to
use stochastic models. For example when the system to be described is based on
the interaction of few molecules, when we are at the presence of a chemical
instability, or when we want to simulate the functioning of a pool of entities
whose compartmentalised structure evolves dynamically. In contrast, stable
metabolic networks, involving a large number of reagents, for which the
computational cost of a stochastic simulation becomes an insurmountable
obstacle, are efficiently modelled with ODEs. In this paper we define a hybrid
simulation method, combining the stochastic approach with ODEs, for systems
described in CWC, a calculus on which we can express the compartmentalisation
of a biological system whose evolution is defined by a set of rewrite rules.
