Introduction. Abnormal thyroid function affect spermato-genesis and male infertility. For men, the aromatase deficiency can cause infertility. In this study, the aim is to investigate the effect of maternal hypothyroidism on offspring testicular morphology and cytochrome-P450-aromatase (P450arom) immunoreactivity. Materials and Methods. Eighteen Wistar albino pregnant rats were divided into three groups, namely A, B and K groups. Hypothyroidism was induced by adding 0.01% of propyl thiouracil (PTU) in drinking water. Hypothyroid mothers, group A: given PTU for 21 days during pregnancy, group B: given PTU for 21 days prior to pregnancy; control mothers, group K, given only water. Hypothyroid and control group mothers' pups at postnatal day (PND) 15 and 60 were sacrificed. We determined immunoreactivity intensity of P450arbm and mRNA levels by RT-PCR performed in the testis tissues. ELISA method was used for thyroid function tests for T3, T4 and TSH. Structure of seminiferous tubule was evaluated by hematoxylin-eosin staining. Results. It was seen that the aromatase expression in 15-day-old maternal hypothyroid groups was similar to the one in the control group while there was a decline in the aromatase expression of 60-day-old groups. As for mRNA, it was determined that it had a tendency to increase over time in all groups but this increase was not significant. The tubule diameter and Johnsen's Testicular Biopsy Score diminished in all hypothyroid groups in comparison to the control group. Conclusion. The changes that occur in the early period of testis development due to maternal hypothyroidism negatively affect testis development in the next stages of life. This situation leads to a decline in aromatase expression in the following years.