The ternary fluorescent complexes of nucleic acids/8-hydroxyquinoline/ lanthanum (III) were studied. Nucleic acids in the study involve natured and thermally denatured calf thymus DNA, fish sperm DNA and yeast RNA. In the range of pH 8.0–8.4 (controlled by NH3-NH4Cl buffer) ternary fluorescent complexes are formed which emit at 485.0 nm for calf thymus DNA and at 480.0 nm for yeast RNA (when excited at 267.0 nm) and emits at 483.0 nm for fish sperm DNA when excited at 265.0 nm. Based on the fluorescence reactions sensitive fluorometric methods for nucleic acids were proposed. Using optimal conditions, the calibration curves were linear in the range of 0.4–3.6 μg˙ml−1for calf thymus DNA, 0.4–4.0 μg-ml−1for fish sperm DNA and 0.4–4.0 μg˙ml−1for yeast RNA, respectively. The limits of determination (3σ) were 0.076 μg˙ml−1for calf thymus DNA, 0.068 μg˙ml−1for fish sperm DNA and 0.329 μg˙ml−1for yeast RNA, respectively. Five synthetic samples were determined with satisfaction.