Rat brain ribonucleases (RNases) were studied. Three types of RNases with maximum activities at pH 5.0, 7.2 and 9.5 were found. The activity of the pH 7.2 enzyme can be detected only by avoiding the interference of a very active inhibitor with p-chlor-mercuri-benzoic acid (PCMB). The effect of bivalent cations (Ca2+, Mg2+), Na+ and ethylenediamine tetraacetic acid (EDTA) was investigated. The activities studied showed a different subcellular distribution. Changes in RNase activity during postnatal rat brain development were studied. The pH 7.2 and 9.5 enzymes have a similar behavior increasing up to the 15th-20th day and remaining constant thereafter. The pH 5.0 enzyme remains constant from the 5th to the 20th day, decreasing thereafter.