In hypocotyls of dry jack pine seeds, RNA was localized mainly in nucleoli and throughout nuclei. After 4 days of imbibition, RNA increased in the cytoplasm and was concentrated particularly in phragmosomes during the first wave of cell divisions. At 5-6 days, when radicles had emerged, uracil-5 or 6-3H was readily degraded to β-alanine via dihydrouracil and β-urcidopropionic acid. The remaining 10% of the radioactivity was recovered mainly as uracil from ribosomal RNA. Polyacrylamide gel electrophoresis was used to separate RNA into ribosomal and transfer RNA fractions. Within 90 min up to 70% of the tritium incorporated into RNA was recovered from 18 and 25 S ribosomal RNA (0.7 and 1.3 × 106 daltons). At the shoot apex, radioactivity per unit area was localized nearly 2-fold more in nuclei than in the cytoplasm and particularly in nuclei from the anneau initial (peripheral zone). At later exposure times to uracil, more radioactivity was detected in the cytoplasm and throughout the shod apex.