'Romerillo' (Baccharis coridifolia DC), 'duraznillo negro' (Cestrum parqui L'Herit.), and 'sunchillo' (Wedelia glauca (Ort.) Hoff.) are highly toxic species producing important economic losses of livestock in Argentina. This study assessed the accuracy and precision in the estimation of the percentage and the mass of these species in the ingesta and feces of sheep experimentally poisoned. This study also evaluated whether the quantified percentage and the calculated mass of each toxic species in the rumen+reticulum, the easiest region to sample, are good estimates of their relative consumption. Results indicate that if species fragment density is quantified, and the percentages of non recognized fragments of the toxic species in their in vitro digestion residues are accounted for (attributing some proportion of the unidentified fragment pool to the target species), estimations are accurate, but their precision differ among species. For a 3 sheep sample, the average mass estimated by microhistological analysis represented 92.3 +/- 5.8 (romerillo), 96.5 +/- 17.3 (duraznillo negro), and 92.0 +/- 12.5% (sunchillo) (P 0.05) from those in the rumen+reticulum. For the evaluated species, the microhistological analysis of the rumen+reticulum not only confirmed the ingestion of the toxic species, but also adequately estimated the percentage in which they were ingested. DOI:10.2458/azu_jrm_v56i4_cid