The investigation concerns the problem of the function of the Telencephalon in the reproductive behaviour of Gasterosteus aculeatus males. This behaviour involves aggressive, sexual and parental activities whose strength was measured in breeding cycles some weeks or months after small parts of the Telencephalon had been removed and compared with non-operated animals. 1. Removal of the N.n. olfactorii or of the Bulbi olf. does not change the normal behaviour. 2. Removal of one complete Telencephalon-hemisphere also does not change the normal behaviour. 3. Removal of the frontal parts of the Telencephalon, including the olfactory bulbs, produces irreversible modifications in the behaviour in subsequent breeding cycles consisting of 1) significantly low Aggression, 2) Sex activity of significantly short duration and 3) significantly high Parental care. 4. Removal of the bilateral parts of the Telencephalon in a breeding cycle one month after the operation produces broadly the same modifications as after frontal operations, but after two or three months the behaviour can revert to the normal pattern. 5. After a transverse cut between the olfactory bulbs and the commissura anterior the behaviour can revert to normal in as little as one month. In other words these changes are reversible. 6. The recovery of the normal pattern is accompanied by regeneration, but is not proportional to the actual amount of tissue regenerated. It happens that recovery of the normal pattern in some days of the breeding week is accompanied by a modified pattern and a striking antagonism between the tendencies in other days. Since this runs sometimes parallel with apparent total regeneration, sometimes with partial regeneration it is probable, that architectonic structures are involved. 7. The regeneration after lateral operation is histologically visible after 42 days as a thick stratum of neuroblasts in the periphery and after 84 days as delamination of the same kind as can be observed in normal animals, and regeneration of nerve fibres. The Telencephalon of Gasterosteus males is concerned with nest-building behaviour and also with governing the relative balance of aggressive, sexual and parental behaviour. This integration is apparently of survival value, ensuring that the animals do obtain, defend and care for the optimal number of clutches per nest.