Effects of cooling and supplemental recombinant bovine somatotropin (rbST) on hemato-biochemical characteristics were studied at different stages of lactation of crossbred Holstein Friesian cows in a tropical environment. Ten primiparous cows were divided into two groups of five animals each. The first group was housed as the non-cooled animals in an open-sided barn with a tiled roof in a normal shaded house (NS), while the second group was housed as cooled cows in an open-sided barn with a tiled roof under misty fan cooling (MFC). Three injections with rbST (500 mg per dose) at each stage of lactation (early, mid and late lactation) significantly increased total milk yield as compared with pretreatment in both cooled and non-cooled cows. Milk fat was significantly increased, while total solids, solid not fat, milk protein and lactose were not affected by the rbST treatment. Hematological parameters, plasma proteins, albumin, glucose, triglyceride, cholesterol, creatinine, alkaline phosphatase (ALP), plasma inorganic phosphate and the activities of plasma aspartate aminotransferase (AST) and alanine aminotransferase (ALT) were not affected by supplemental rbST in cooled and non-cooled cows. Supplementation of rbST caused a significant decrease in plasma urea concentration, while plasma FFA concentrations significantly increased in both cooled and non-cooled cows. The results of the present study suggest that exogenous rbST is efficacious in increasing milk yield without adverse effects on lactating crossbred Holstein cows in a tropical environment.