Summary. At 3-month intervals from birth to 27 months of age, 3 female armadillos were killed. The number and size of follicles >202 μm were determined, plasma progesterone concentration was measured, and values were correlated with age. Blood samples were taken monthly by femoral vein puncture and plasma was analysed by radioimmunoassay for progesterone concentration. At necropsy, both ovaries were visually inspected for a corpus luteum, weighed and then processed for routine histology. The number of normal, antral follicles >202 μm were counted. These follicles were arbitrarily categorized into 15 different size groups (every 77μm). Total number of follicles >202 μm varied from 15·5± 1·3 at 15 months of age to 26·3 ± 1·9 at 21 months. Follicles of a size (>978 μm) most likely to ovulate were present only at ≥9 months of age. Progesterone values remained below the adult concentration (5 ng/ml) until 15 months of age. A concentration of progesterone indicative of ovulation (∼ 10 ng/ml) occurred between 17 and 20 months of age. The findings of the present study demonstrate that the female armadillo is reproductively mature after 15 months of age.