Abstract. Constraints on the Seifert invariants of orientable 3-manifolds M which admit fixed-point free S 1 -actions and embed in R 4 are given. In particular, the generalized Euler invariant of the orbit fibration is determined up to sign by the base orbifold B unless H 1 (M ; Z) is torsion free, in which case it can take at most one nonzero value (up to sign). An H 2 × E 1 -manifold M with base orbifold B = S 2 (α 1 , . . . , α r ) where all cone point orders are odd embeds in R 4 if and only if its Seifert data S is skew-symmetric.The question of which closed 3-manifolds M (other than homology spheres) embed in R 4 has received surprisingly little attention. (The relevant papers known to us are [2,3,[5][6][7][8][9][10][11].) In particular, it is not yet known which Seifert fibred 3-manifolds embed, although in many other respects this is a well-understood class of spaces, with natural parametrizations in terms of Seifert data. It was shown earlier that if M embeds in R 4 , then it must be orientable and the torsion subgroup T (M ) of H 1 (M ; Z) must be a direct double: T (M ) ∼ = U ⊕ U for some finite abelian group U [8]. Moreover, the linking pairing M on T (M ) must be hyperbolic [13]. Most of the known constructions give smooth embeddings, but these conditions must also hold if M embeds as a TOP locally flat submanifold.The first two sections establish our notation and summarize some basic facts about compact 3-manifolds in R 4 . In §3 we shall observe that T (M ) being a direct double imposes strong constraints on the Seifert data of orientable 3-manifolds which admit fixed-point free S 1 -actions and which embed in R 4 . The present simple argument does not work for orientable Seifert fibred 3-manifolds with nonorientable base orbifolds. These admit no compatible S 1 -action. However in [3] we used the Z/2Z-index theorem to constrain the Euler invariants for such 3-manifolds. As a consequence we were able to settle there the question of embeddability for manifolds having one of the geometries S 3 , E 3 , Nil 3 , S 2 × E 1 or Sol 3 . When the Seifert data is "skew-symmetric" (i.e., is a set of complementary pairs) and all cone point orders are odd, the corresponding Seifert manifold embeds smoothly [3]. Such a manifold has generalized Euler invariant ε = 0 and so is geometric of type E 3 , S 2 × E