Effects of stachytarpheta cayennensis on haematological parameters were investigated in thirty (30) albino rats for a period of two (2) weeks. There was significant (P<0.05) decline in red blood cells values with increase in extract concentration as compared with control but the values were within the normal range. There was increase in packed cell value with increase in extract dosage but the difference was not significant (P> 0.05) except in high dose that showed low value with increase dosage concentration and was significant (P< 055) as compared with control. There was little increases in haemoglobin value as in low and medium dosages than control but a drop at high dosage of the extract which was significant P<0.05. The mean corpuscular haemoglobin, (MCH) mean corpuscular volume, (MCV) and mean corpuscular haemoglobin concentration, (MCHC) were all reduced with increases concentration of the extract. But, the white blood cells and the platelets were all increased with increased extract concentration. However, there was a drastic increase in the weight of the rats at increasing dosages of the extract particularly at 14 days of the administration. It is observed that extracted of S. cayennensis does not impact positively on blood cells and it is dangerous in the treatment.