Context The use of progesterone vaginal implants in sheep is frequently associated with vaginitis and adherences; and can hamper fertility in estrus synchronization programs. Aims The aim of this study was to characterize the plasma hormone profile induced by two injectable progesterone (iP4) formulations in ewes. Methods In Experiment 1, ewes received 20 or 40 mg of an iP4, with a third group as a control. In Experiment 2, 75 or 150 mg of a long-acting iP4 were administered, again with a control group. Key results In Experiment 1, progesterone concentrations remained at luteal levels for less than 24 h. The intervals from iP4 treatment to the onset of estrus and ovulation were greater in treated than in control ewes (P < 0.01). The proportion of ewes that ovulated within 8 days after treatment was lower in ewes that received 40 mg iP4 than in control ewes (P < 0.05). In Experiment 2, mean progesterone concentrations remained at luteal levels for 120 h. The interval from iP4 administration to estrus was longer in 150 mg treated ewes than in control ewes (P < 0.01). The interval from treatment to ovulation was longer in ewes treated with 150 mg than 75 mg and control ewes (P < 0.05). The proportion of ewes that ovulated was lower at 150 mg (P < 0.05). Conclusions The iP4 formula used in Experiment 1 could not maintain luteal levels for more than 24 h. The use of long-acting iP4 maintained plasma progesterone concentrations above luteal levels for at least 5 days. The long-acting treatment can therefore be tested for estrous synchronization treatments. Implications The long-acting progesterone may be an alternative to short estrous synchronization protocols.