An experiment was designed to determine plasma progesterone (P4) concentrations in ovariectomized cows treated with either CIDR-B devices (Pfizer Animal Health, Hamilton, New Zealand) impregnated with 1.9 g of P4 or DIB devices (Syntex, Argentina) impregnated with 0.5 or 1.0 g of P4. Fifteen ovariectomized beef cows, weighing 350 to 450 kg and with a body condition score (BCS) 2.5 to 3.0 out of 5 were used. All cows were ovariectomized 30 days before the beginning of the trial and had plasma P4 concentrations <1 ng/mL (measured by radioimmunoassay (RIA)) 10 days before the trial. On Day 0, cows were stratified by weight and randomly assigned to one of three treatment groups to receive intravaginal devices containing 0.5 g of P4 (DIB 0.5), 1 g of P4 (DIB 1.0), or 1.9 g of P4 (CIDR-B). Blood samples were taken 12 and 24 h after intravaginal device insertion and then every 24 h until device removal on Day 7. Jugular blood samples were drawn into heparinized tubes and centrifuged within 20 min of collection; the plasma was harvested, frozen, and stored at -10^°C. Progesterone was measured in plasma samples using a commercial solid-phase RIA (Coat-a-count; Diagnostic Products Corp., Los Angeles, CA, USA). All of the samples were analyzed at one time and the intra-assay coefficient of variation was 13%. Plasma P4 concentrations were compared among treatments by calculating the area under the curve over time and testing for differences by ANOVA. Plasma P4 profiles were also analyzed by ANOVA for repeated measures. Analysis of plasma P4 profiles revealed a day effect (P < 0.001) but not a treatment effect (P > 0.13) or a day by treatment interaction (P > 0.16; Table 1). The area under the curve did not differ (P = 0.95) among groups (DIB 0.5: 2236.4 ng, DIB 1.0: 2164.6 ng, and CIDR-B: 2266.8 ng). In all treatments, plasma P4 profiles were characterized by a sudden increase in P4 concentrations within 12 h of device insertion and a gradual decrease during the following several days. It was concluded that the use of CIDR-B devices impregnated with 1.9 g of P4 or DIB devices impregnated with 1.0 or 0.5 g of P4 did not result in different plasma P4 concentrations in ovariectomized cows. Further studies are needed to confirm these results in lactating and dry intact (not ovariectomized) cattle.