The objective of this study was to compare the influence of injectable altrenogest on uterine tone (UT) and embryonic vesicle growth (EVG) in non-cyclic recipient mares in anestrus and transition with cyclic mares. This study was conducted in Brazil in the winter season (June to September) between 2011 and 2014. One-hundred fifty-five embryo transfers (ET) were made in crossbreed mares from 4 to 10 years of age, between 350 and 450 kg of weight. The recipient mares were distributed into 3 groups: G1: anestrus (n = 67), mares with follicles less than 15 mm and absence of corpus luteum; G2: transition (n = 14), mares with follicles between 15 and 20 mm and absence of corpus luteum; and G3: cyclic (n = 74), mares with follicles larger than 20 mm and presence of corpus luteum. On Day 0 (day of donor mare ovulation), recipient mares in G1 and G2 received a single IM dose of 17β-oestradiol (10 mg). Recipients were examined by ultrasound on Days 1 and 2 to evaluate the presence of uterine oedema. On Day 2, once oedema was confirmed, recipients were given 180 mg of altrenogest IM, and altrenogest treatment was repeated every 8 days until Day 90 of gestation. The G3 recipient mares were not treated with 17β-oestradiol and altrenogest. For mares in G1 and G2, ET was performed 6 days after the initiation of altrenogest treatment. For recipients in G3, ET was performed between Days 6 and 8. Uterine tone was evaluated before ET on a scale of 0 to 3 (tonus absent 0, and tonus intense 3). Conception rate was determined by ultrasound at Day 15. The diameter of the EVG was calculated by measuring the length and width of the embryonic vesicle on Days 15, 20, 25, 30, 35, 40, and 45 using ultrasonography. Analysis of the variables UT and EVG was performed with Proc Mixed. Results are represented as ± standard error of the mean. Statistical analysis for conception rate was made by chi-square distribution adjusted with the Fisher’s test when it was required. All data were analysed with the SAS statistical package. There was no difference (P > 0.05) in UT between recipient groups (G1: 2.6 ± 0.04; G2: 2.6 ± 0.09; G3: 2.5 ± 0.04). In addition, there was no effect of recipient group nor an interaction between recipient group and day of gestation for EVG (P > 0.05). However, conception rate was greater (P < 0.05) in G2 (11/14 = 78%) and G3 (44/74 = 59%) compared with G1 (26/67 = 39%). Results of the present study demonstrate the effectiveness of injectable altrenogest for use in managing anestrus and transition mares for ET.