With the aim of enhancing the biological activity of ruthenium-nitrosyl complexes, new compounds with four equatorially bound indazole ligands, namely, trans-[RuCl(Hind)(4)(NO)]Cl-2 center dot H2O ([3]Cl-2 center dot H2O) and trans-[RuOH(Hind)(4)(NO)]Cl-2 center dot H2O ([4]Cl-2 center dot H2O), have been prepared from trans-[Ru(NO2)(2)(Hind)(4)] ([2]). When the pH-dependent solution behavior of [3]Cl-2 center dot H2O and [4]Cl-2 center dot H2O was studied, two new complexes with two deprotonated indazole ligands were isolated, namely [RuCl(ind)(2)(Hind)(2)(NO)] ([5]) and [RuOH(ind)(2)(Hind)(2)(NO)] ([6]). All prepared compounds were comprehensively characterized by spectroscopic (IR, UV-vis, H-1 NMR) techniques. Compound [2], as well as [3]Cl-2 center dot(2)(CH3)(2)CO, [4]Cl-2 center dot 2(CH3)(2)CO, and [5]center dot 0.8CH(2)Cl(2), the latter three obtained by recrystallization of the first isolated compounds (hydrates or anhydrous species) from acetone and dichloromethane, respectively, were studied by X-ray diffraction methods. The photoinduced release of NO in [3]Cl-2 and [4]Cl-2 was investigated by cyclic voltammetry and resulting paramagnetic NO species were detected by EPR spectroscopy. The quantum yields of NO release were calculated and found to be low (3-6%), which could be explained by NO dissociation and recombination dynamics, assessed by femtosecond pump-probe spectroscopy. The geometry and electronic parameters of Ru species formed upon NO release were identified by DFT calculations. The complexes [3]Cl-2 and [4]Cl-2 showed considerable antiproliferative activity in human cancer cell lines with IC50 values in low micromolar or submicromolar concentration range and are suitable for further development as potential anticancer drugs. p53-dependence of Ru-NO complexes [3]Cl-2 and [4]Cl-2 was studied and p53-independent mode of action was confirmed. The effects of NO release on the cytotoxicity of the complexes with or without light irradiation were investigated using NO scavenger carboxy-PTIO.