Tonin- and kallikrein-like activities were investigated in different regions of the rat brain. The highest values of specific tonin activity, expressed as picomoles of angiotensin II liberated per minute per milligram of protein, were found in the neurohypophysis (359 +/- 190) and in the archicerebellum (200 +/- 68). The highest level of total tonin activity (picomoles of angiotensin II liberated per minute) was observed in the archicerebellum (902 +/- 308) which retained 97% of total tonin activity of whole cerebellum. Tonin activity was not detected in the cortex of cerebellum and in the choroid plexus. Low to intermediate values of specific (1.09 +/- 0.33 to 5.32 +/- 2.37) and total (1.38 +/- 0.55 to 93.00 +/- 49.30) tonin activity were observed in adenohypophysis, cerebellar nuclei, hypothalamus, thalamus, midbrain, pons, medulla and neurohypophysis. The lowest values of specific (0.11 +/- 0.05) and total (0.69 +/- 0.31) activities were observed in the hippocampus. Kallikrein-like activity was expressed as picomoles of p-nitroaniline liberated per minute per milligram of protein. No activity was detected in the neurohypophysis. For other regions, the values of the specific activity ranged between 72 +/- 18 and 282 +/- 14 except for the choroid plexus which was 5 +/- 2. The total kallikrein activity was also homogeneous ranging from 330 +/- 100 to 1870 +/- 112. For the choroid plexus and adenohypophysis the total kallikrein activity was 2.0 +/- 0.8 and 27 +/- 11, respectively.