A lack of thyroid hormone, i.e. hypothyroidism, during early development results in multiple morphological and functional alterations in the developing brain. In the present study, behavioural effects of perinatal and chronic hypothyroidism were assessed during development in both male and female offspring of hypothyroid rats. To induce hypothyroidism, dams and offspring were fed an iodide-poor diet and drinking water with 0.75% sodium perchlorate; dams starting 2 weeks prior to mating and pups either until the day of killing (chronic hypothyroidism) or only until weaning (perinatal hypothyroidism) to test for reversibility of the effects observed. Neuromotor competence, locomotor activity and cognitive function were monitored in the offspring until postnatal day 71 and were compared with age-matched control rats. Early neuromotor competence, as assessed in the grip test and balance beam test, was impaired by both chronic and perinatal hypothyroidism. The open field test, assessing locomotor activity, revealed hyperactive locomotor behavioural patterns in chronic hypothyroid animals only. The Morris water maze test, used to assess cognitive performance, showed that chronic hypothyroidism affected spatial memory in a negative manner. In contrast, perinatal hypothyroidism was found to impair spatial memory in female rats only. In general, the effects of chronic hypothyroidism on development were more pronounced than the effects of perinatal hypothyroidism, suggesting the early effects of hypothyroidism on functional alterations of the developing brain to be partly reversible and to depend on developmental timing of the deficiency.