--- - |2+ Abstract Aim Besides their metabolic and endocrine functions, the growth hormone (GH) and its mediated factor, the insulin‐like growth factor I (IGF‐I), have been implicated in different brain functions, including neurogenesis. Long‐lasting elevated GH and IGF‐I levels result in non‐reversible somatic, endocrine and metabolic morbidities. However, the subcutaneous implantation of the GH‐secreting (GH‐S) GC cell line in rats leads to the controllable over‐secretion of GH and elevated IGF‐I levels, allowing the experimental study of their short‐term effects on brain functions. Methods Adult rats were implanted with GC cells and checked 10 weeks later, when a GH/IGF‐I‐secreting tumour was already formed. Results Tumour‐bearing rats acquired different operant conditioning tasks faster and better than controls and tumour‐resected groups. They also presented better retentions of long‐term memories in the passive avoidance test. Experimentally evoked long‐term potentiation (LTP) in the hippocampus was also larger and longer lasting in the tumour bearing than in the other groups. Chronic adult‐onset of GH/IGF‐I hypersecretion caused an acceleration of early progenitors, facilitating a faster neural differentiation, maturation and integration in the dentate gyrus, and increased the complexity of dendritic arbours and spine density of granule neurons. Conclusion Thus, adult‐onset hypersecretion of GH/IGF‐I improves neurocognitive functions, long‐term memories, experimental LTP and neural differentiation, migration and maturation. - 'Acta Physiologica, EarlyView. '