We investigated the in vivo effects of acetylsalicylic acid, diclofenac, indomethacin, ibuprofen, and ketoprofen on the concentrations of various blood constituents. Total protein, glucose, calcium, and inorganic phosphate were not significantly affected by any of these drugs. Ketoprofen had no definite influence on any constituent. Acetylsalicylic acid induced an increase in cholesterol, triglyceride, and iron; albumin, uric acid, and creatinine decreased with ibuprofen therapy. Urea nitrogen increased in patients treated with diclofenac or indomethacin. Our protocol for the study of in vivo drug effects is discussed.