Background: Circulating prolactin concentration in rodents and humans is sexually dimorphic. Oestrogens are a well-characterised stimulator of prolactin release. Circulating prolactin fluctuates throughout the menstrual/oestrous cycle of females in response to oestrogen levels, but remains continually low in males. We have previously identified androgens as an inhibitor of prolactin release through characterisation of males of a mouse line with a conditional pituitary androgen receptor knockout (PARKO) which have an increase in circulating prolactin, but unchanged lactotroph number. Objectives: In the present study, we aimed to specify the cell type that androgens act on to repress prolactin release. Materials and methods: PARKO, lactotroph-specific, Pit1 lineage-specific and neural-specific conditional androgen receptor knockout male mice were investigated using prolactin ELISA, pituitary electron microscopy, immunohistochemistry and qRT-PCR. Results: Lactotroph-specific, Pit1 lineage-specific and neural-specific conditional AR knockouts did not duplicate the high circulating prolactin seen in the PARKO line. Using electron microscopy to examine ultrastructure, we showed that pituitary androgen receptor knockout male mice develop lactotrophs that resemble those seen in female mice. Castrated PARKO males have significantly reduced circulating prolactin compared to intact males. When expression of selected oestrogen-regulated anterior pituitary genes was examined, there were no differences in expression level between controls and knockouts. Discussion: The cell type that androgens act on to repress prolactin release is not the lactotroph, cells in the Pit1-lineage, or the dopaminergic neurons in the hypothalamus. PARKO males develop a female-specific lactotroph ultrastructure that this is likely to contribute to the increase in circulating prolactin. Castrated PARKO males have significantly reduced circulating prolactin compared to intact males, which suggests that removal of both circulating oestrogens and androgens reduces the stimulation of pituitary prolactin release. Conclusion: Further investigation is needed into prolactin regulation by changes in androgen-oestrogen balance, which is involved sexual dimorphism of development and diseases including hyperprolactinemia.