We denote by Sp(n) [resp. Sp(n, lR)] the compact (resp. split) real form of the symplectic group of degree n (size 2n). Then there are two natural embeddings: (i) (Sp(1) x Sp(n))/+ (I, 1)c. SO(4n) (the compact orthogonal group) and (ii) Sp(r, IR) x SO(4n) % Sp(4rn, IR) (a dual reductive pair [8]; r= 1,2, ...).From the theory of the Weft representations and that of Howe's dual reductive pairs, one anticipates a global correspondence between automorphic forms, F on Sp(1) x Sp(n) and OF on Sp(r, IR), "defined" by the above two embeddings via theta series. The aim of this paper is to study such a correspondence, giving explicit relations between their L-functions L(s, F) and L(s, 0F) for the case n > 2 and r = 1. This generalizes [10] which was for (n, r) = (2, 1). We note that the Saito-Kurokawa lifting can be regarded as a split symplectic version of [10]. We also add that Yoshida [20] studied the case (n, r) = (1, 2), and Tanigawa [18], the case (n, r) = (1,3). Thc global algebraic group that we consider here iswhere B is a definite quaternion algebra over the rationals Q and GR,, is the group of similitudes of the standard positive quaternion hermitian metric on B': G n,, = { g e G L,( B); g-tg = n(g)l, with some n(g) ~ Q • }.* The first named author was partially supported by Max Planck Institute fiir Mathematik and SFB 40, during the preparation of this paper Incidentally, this restriction on the weight follows automatically from the assumption r= 1 [! 1]. For each such automorphic form F on G A we associate its "theta series" Ov which is a form on GL(2)A. Our first main result (Theorem 1, Sect. 2) gives an explicit relation between two L-functions L(s, F) and L(s, Or).There are examples of F with 0 v + 0 even for n > 3 (cf. Sect. 3). Our standard choice of the level makes it easy to treat automorphic forms in a "classical manner". For example, the study of Hecke operators acting on forms (spherical functions) is reduced to analysis of their dual actions on "invariant 0-cycles" on some B-spaces. Our second result (Theorem 2, Sect. 2) describes the eigenvalues of Hecke operators for the eigenfunction F by the sum of special values of F on invariant 0-cycles. The Proof of Theorem 2 will be given in Sect. 4. In Sect. 5, we first give some refinement of Theorem 2, and then we prove Theorem I by using this and applying the commutation relation of Hecke operators and the Siegel ,b-operators (at good non-archimedean places). We note that the pair (Sp(l) • Sp(n), Sp(r, lR)) is not a dual reductive pair for n>3, even in the weak sense of Kudla [13]. The existence of a "good" global correspondence even in such a case may suggest that we can use the theory of the Weil representations for a wider class of reductive groups. It is the authors' desire to draw attentions to this by showing such an example, that motivated us to publish these results. It seems to be an interesting problem to generalize our results starting from other subgroups G' C SO(n') [instead of the above embedding (i)]. In this case, the main problem would...