Abundant ectopic granule cells scattered in the cerebellar molecular layer have been observed in 30-day-old hypothyroid rats. Their morphological features indicate that they must be regarded as mature heterotopic cells arrested during their migration towards the granular layer. As their impoverished dendritic trees are identical to those seen in controls, it is unlikely that the lack of thyroid hormones played a major role in the deficient dendritic outgrowth. The study of 180-day-old hypothyroid rats revealed that although ectopic granule cells remained quite numerous, their number per unit surface was lesser than in the 30-day-old hypothyroid group. This finding may be related to the capacity displayed by heterotopic neurons to establish synaptic contacts with the components of the molecular layer. This was inferred by the presence of a peculiar synaptic cell investment formed by axosomatic and somatodendritic contacts in 180-day-old hypothyroid rats which shows that the surviving ectopic granule cells manage to adapt to an adverse milieu.