Using the rings of Lipschitz and Hurwitz integers H(Z) and Hur(Z) in the quaternion division
algebra H, we define several Kleinian discrete subgroups of P SL(2, H). We define first a Kleinian subgroup
P SL(2, L) of P SL(2, H(Z)). This group is a generalization of the modular group P SL(2, Z). Next we de-
fine a discrete subgroup P SL(2, H) of P SL(2, H) which is obtained by using Hurwitz integers. It contains
as a subgroup P SL(2, L). In analogy with the classical modular case, these groups act properly and dis-
continuously on the hyperbolic quaternionic half space. We exhibit fundamental domains of the actions
of these groups and determine the isotropy groups of the fixed points and describe the orbifold quotients
H 1 H /P SL(2, L) and H 1 H /P SL(2, H) which are quaternionic versions of the classical modular orbifold and
they are of finite volume. Finally we give a thorough study of their descriptions by Lorentz transformations
in the Lorentz-Minkowski model of hyperbolic 4-space.
