Lasing at the r-m transitions in atomic strontium at l = 6.45 mm holds very good promise for medical applications. This lasing is known to occur together with that at the r-m transitions in ion strontium at l = 1.03 and 1.09 mm and, under certain conditions, with the lasing at the transitions from the 4d3D3,2,1 levels to the 5p3P 02,1,0 levels in atomic strontium at l = 3.01, 2.92, 2.69, and 2.60 mm. The possible mechanisms of the population inversion formation for the transitions in atomic and ion strontium are discussed with the allowance made for specific features of the atomic strontium structure and the excitation functions of the singlet and triplet SrI states.