Zeranol (Z) is a widely used growth promotant; however, plasma Z profiles in cattle implanted with Z have not been characterized. This study was conducted to determine bovine plasma Z profiles. In Exp. 1, four steers (BW = 284.8 +/- 5.6 kg) were implanted with 108 mg of Z (Ralgro). To determine the effect of sampling site on plasma Z concentrations, blood was sampled by venipuncture from the maxillary vein ipsilateral (IMV) to the ear in which Z was implanted and from the ipsilateral (IJV) and contralateral (CJV) jugular veins of each steer. Samples were collected on d 1, 4, 6, 8, 11, and 13 after implantation and Z was assayed by RIA. There was an effect of sampling site (P < .01). The overall mean plasma Z concentrations and 95% confidence intervals (CI) for each vessel were 282 (CI: 172 to 463), 135 (CI: 85 to 215), and 67 (CI: 42 to 106) pg/mL for the IMV, IJV, and CJV, respectively. Plasma Z concentration was higher (P < .05) in IMV than in IJV and higher (P < .05) in IJV than in CJV. In Exp. 2, nine steers (BW = 316.7 +/- 10.0 kg) were implanted with 108 mg of Z and IMV blood was collected on d 0, 1, 3, 7, 10, 14, 17, 21, 28, 35, 42, 56, 63, 73, and 91 after implantation. Day affected plasma Z concentration (P < .01); plasma Z was elevated above preimplantation levels for 91 d (P < .05).(ABSTRACT TRUNCATED AT 250 WORDS)