The morphological and behavioral effects of neonatal electrothermal lesions of the mediodorsal thalamus on the development of the prefrontal cortex were studied. Lesions of the mediodorsal nucleus (MDT), inflicted on the day of birth, caused no significant changes in prefrontal architecture on day 35. On the other hand, a significant decrease in cortical width (4.7-7.7%) was observed at some places within the lateral and supragenual parts of the prefrontal cortex. However, these local decreases in cortical width were not reflected by a significant decrease of the total volume of the particular prefrontal subareas. In adulthood, rats with neonatal MDT lesions were exposed to an operant delayed alternation task, which is known to depend upon the integrity of the prefrontal cortex, in order to investigate the behavioral consequences of the lesions for prefrontal functioning. The lesions did not impair the rats abilities to learn the spatial delayed alternation task. Neonatally lesioned and control rats scored equally. Given the relatively mild effects of MDT lesioning, thalamic fibers do not seem to play a crucial role, at least not during the postnatal period of prefrontal cortical development. It is discussed whether or not this is a specific characteristic of agranular association cortex, in which the termination of thalamic and cortical afferents overlap in layer III.