My PhD project Blood, Sweat & Tears studies bodily fluids in medical contexts. I analyse how the development of new scientific instruments and chemical laboratories in 18th-century Dutch Republic changed the way physicians perceived the nature and functions of fluids in physiology and pathology. Simultaneously, however, the specific properties of these fluids enabled or restricted the possibilities of new research. Urine was easily obtainable and could be measured, mixed and distilled. Yet sweat’s volatility was far more difficult for early modern physicians to grasp.