Differences in the reproductive physiology of pregnant and mated non-pregnant koalas were examined to investigate maternal recognition and identify potential circulating pregnancy and/or embryonic loss biomarkers. Koalas were separated into three groups: pregnant (n = 4), mated but non-parturient (n = 4), and gonadotropin releasing hor-mone (GnRH) agonist-treated females (n = 7). Plasma was collected on the day of mating/GnRH injection (D0) and on multiple subsequent days. Progesterone concen-trations were measured by enzyme immunoassay and plasma proteomes were ana-lysed using filter-aided sample preparation followed by liquid chromatog-raphy-tandem mass spectrometry (LC-MS/MS) employing sequential window acquisi-tion of all theoretical fragment ion spectra. Ovulation induction mechanisms influenced peri-ovulatory progesterone secretion (pregnant 40.7  3.3 ng/ml vs GnRH 15.7  1.8 ng/ml), although no significant differences in progesterone occurred later in the luteal phase. LC-MS/MS identified 158 proteins, representing the first koala plasma proteo-me. Leucine-rich -2-glycoprotein (LRG1) was significantly elevated at D2 in pregnant compared to GnRH treated, and pregnant D9 and D19. In pregnant females, fibronectin (FN1) was significantly more abundant at D19 compared to D9 but not significantly different between treatments. These preliminary findings provide the foundational data for further investigation into maternal recognition and pregnancy and/or em-bryonic loss biomarkers in koalas.