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Viscachas are native rodents of South America that present a long pregnancy of ~154 days. In this work, we analysed variations in the expression of proliferating cellular nuclear antigen, oestrogen and androgen receptors (ERα and AR) in pituitary pars distalis (PD) and pars tuberalis (PT) in relation to oestradiol and testosterone serum levels in non-pregnant and pregnant viscachas. In PD, cell proliferation increased with pregnancy and lactotrophs proliferated during mid-pregnancy (MP). ERα nuclear-immunoreactive cells (ERαn-ir) were maximal in late pregnancy and AR expression did not vary during pregnancy. In PT, cell proliferation and AR expression increased during pregnancy, but ERα expression was very scarce. The immunostaining pattern of receptors was different in PD and PT. The peak of serum oestradiol and testosterone occurred during MP. Our results suggest that cell proliferation and gonadal receptors might be differentially regulated in the pituitary by oestradiol and testosterone during viscacha pregnancy. Oestrogens and androgens regulate adenohypophyseal function during pregnancy. In this study, we investigated the relationship between serum levels of gonadal hormones and cell proliferation in pituitary pars distalis and pars tuberalis of non-pregnant and pregnant viscachas. Our results provide the morphological basis for discussing the likely role of gonadal steroids in the histophysiological variations of the pituitary in a rodent with unique reproductive characteristics compared with other mammals. [ABSTRACT FROM AUTHOR]