  We study the phase diagram of spin-triplet superconductors, considering the
effect of the external magnetic field on the electrons' spins. For a given
symmetry of the order parameter and a generic orientation of the field, we find
that the paramagnetic limit for superconductivity diverges at low temperatures.
Furthermore, we identify a range of temperatures where the transition between
normal and superconducting phases becomes of the first order. When two
tricritical points exist along the transition line, a first order phase
transition between two superconducting phases may develop in vicinity of the
tricritical point with lower temperature. We discuss the implications of our
findings for the anisotropy of the upper critical field in UPt$_3$, a candidate
material for triplet superconductivity, when both the paramagnetic and orbital
effects are taken into account.
