A hydrogel forming system which comprises a hydrophobic macromer with unsaturated group terminated ends and a hydrophilic polymer which is a polysaccharide (e.g., dextran) containing hydroxy groups which are reacted with unsaturated group introducing compound, is convertible by free radical polymerization to form a hydrogel containing a three dimensional crosslinked polymer network containing hydrophobic and hydrophilic components. Agent can be entrapped in the polymer network, e.g., drugs, macromolecules or synthetic or natural polymers, for controlled release therefrom.