The objective of this study was to determine if the administration of poppy seeds to horses would result in detectable concentrations of morphine in urine and blood samples, as has been shown to occur in humans. In this study blood and urine samples were collected following administration of poppy seeds and morphine sulfate orally to four horses. Urine samples were subjected to enzyme-linked immunosorbent assay (ELISA) for the presence of morphine. All urine samples testing positive by ELISA, as well as plasma samples collected after administration of the 10-g doses of poppy seeds, were analyzed by gas chromatography–mass spectrometry for the presence of morphine. Morphine was detectable in the plasma samples for at least 4 h after administration of 10 g of poppy seeds. Morphine was detectable in urine samples for up to 24 h after administration of 10 g, 5 g, and 1 g of poppy seeds and 426.7 µg of morphine as morphine sulfate. The results of this study indicate that horses that consume or are administered poppy seeds may have detectable concentrations of morphine in their urine and plasma for hours after administration.