The XTGe spin-lattice relaxation time (T1) and spin-spin relaxation time (T2) in such alkylgermanes as symmetrical GeR4 (R = methyl, ethyl, n-propyl, n-butyl) and the unsymmetrical germacyclohexane derivatives 1,1,3-trimethyl-, 1,1,4-trimethyl-, and 1,1,3,5-tetramethyl-1-germacyclohexane have been measured at several temperatures in order to elucidate the relaxation mechanism of germanium nuclei in organogermanium compounds. Furthermore, the viscosities of the solutions of symmetrical alkylgermanes were measured to obtain the reorientational correlation times. T1 and T2 are the same for all the compounds investigated at room temperature, and these become shorter as the molecular radii become larger. The relaxation times of unsymmetrical germacyclohexane derivatives are even shorter than those of symmetrical GeR4. T1 and T2 of germanium nuclei in alkylgermanes were found to be dominated by the quadrupole relaxation. The reorientational correlation times ((0.69-10.9) x 10 s) and the activation energies (9.6-13.2 kJ/mol) for GeR4 are also discussed. 35 references, 5 figures, 2 tables.