The cardiovascular effects of constant rate infusion (CRI) of S(+)-ketamine in dorsally recumbent halothane-anesthetized horses were assessed. Six mixed-breed, adult, male horses, weighting 350–450 kg were used. The animals were randomly distributed into two (treatments) groups, with each horse receiving both treatments. Sedation with xylazine (1 mg/kg IV), infusion of 10% guaifenesin (100 mg/kg in 5% glucose), induction with S(+)-ketamine (1 mg/kg IV), and maintenance with halothane (end-tidal concentration of 1.5 minimal alveolar concentration [MAC]) was standardized for both groups. When halothane end-tidal concentration stabilized at 1.5 MAC, CRI with S(+)-ketamine (GrKet) at 0.01 mg/kg/min (diluted in 250 mL of 0.9% saline solution) or the same volume of 0.9% saline solution (GrSal) was initiated (M0). Constant rate infusion was maintained for 50 minutes (M50). Cardiac output (CO), fractional shortening (FS) and ejection fraction (EF), heart rate (HR), respiratory rate (ƒR), systolic arterial pressure (SAP), mean arterial pressure (MAP) and diastolic arterial pressure (DAP) were recorded at baseline (B); lateral recumbency (Rec); 2-minute postanesthetic induction (PI); beginning of CRI (M0); CRI elapsed time (M10–M50). Only MAP differed between groups (M20). ƒR decreased (P ≤ .05) PI in GrKet and during CRI in GrSal. Cardiac output, FS, EF, SAP, MAP, and DAP decreased (P ≤ .05) in both groups during CRI. Constant rate infusion with S(+)-ketamine at 0.01 mg/kg/min was ineffective in improving cardiocirculatory depression commonly observed in halothane-anesthetized horses. Despite possible limitations, transcutaneous echocardiographic assessment of left ventricular activity in dorsally recumbent horses seemed applicable. Further studies are encouraged to validate its reliability.