Monitoring of changes in some blood laboratory parameters in 34 horses after ingesting freshly harvested maize that was to be used for ensiling, heavily contaminated with young Datura stramonium plants, is described. For a 7-day period the following parameters were monitored: haemoglobin content (HGB), red blood cell counts (RBC), white blood cell counts (WBC), haematocrit (HCT), mean corpuscular volume (MCV), mean corpuscular haemoglobin (MCH), mean corpuscular haemoglobin concentration (MCHC), differential white cell counts (DWC), erythrocyte sedimentation rate (ESR), protein fractions, aspartate aminotransferase (AST), lactate dehydrogenase (LDH), total bilirubin (TB), direct bilirubin (DB), blood glucose (Glu), total protein (TP), globulin (Glob) and albumin (Alb). The intoxication was accompanied by erythrocytosis, leukocytosis, regenerative left shift neutrophilia, lymphopaenia, eosinopaenia, increased haematocrit values, low erythrocyte sedimentation rate, hyperglycaemia, bilirubinaemia, hypoproteinaemia and increased activity of AST and LDH. No changes occurred in the mean corpuscular volume (MCV), mean corpuscular haemoglobin (MCH), mean corpuscular haemoglobin concentration (MCHC), albumin, globulin and globulin fractions (alpha 1, alpha 2, beta 1, beta 2 and gamma). The blood parameters returned to normal between post-intoxication days 2 and 5. The observed changes in clinical chemistry indices could be used in the diagnosis, differential diagnosis and prognosis of Jimson weed intoxication.