This work is a study of the relation between the vanishing of Ext functor and the existence of regular bases in the cartesian product and tensor product of some special Kothe spaces. We give some new results concerning Sg Spaces in Chapter 3 and the study in the last chapter is about the existence of pseudo-regular bases in the cartesian product and tensor product of two regular Schwartz Kothe spaces E and F , one of which having property (DN), when Ext(E x F,E x F) vanishes.