In the present randomized single-blind study local and systemic tolerability of alpha-methyl-4-(2-thienylcarbonyl)-phenyl acetic acid (suprofen, Suprol) 200 mg/ml i.m. were compared with those of diclofenac 25 mg/ml and placebo. The three treatment groups consisted of 15 patients each and were homogeneous with respect to demographic parameters. The volunteers were assigned to these groups in random fashion and underwent treatment with 3 injections/day of either suprofen or diclofenac or placebo for 7 days. Mild local pain or systemic intolerance signs were equally rare in all three groups. The laboratory tests with the three preparations studied failed to indicate any negative influence on the hematopoietic organs or the adrenal activity. Abnormal changes in the ECG and the cardiovascular parameters during the treatment were not observed. Moderately or slightly elevated SGOT and SGPT values were seen during treatment with suprofen; however, these values returned to normal after the treatment. As compared with these elevations the values for subjects on diclofenac were higher and did not return to normal within 5 days following withdrawal of the drug. Moreover, the creatine phosphokinase activity, observed in both the suprofen and the diclofenac group, was so extremely high in subjects on diclofenac that this drug had to extremely high in subjects on diclofenac that this drug had to be withdrawn on day 4 of the study. On the basis of these results it can be stated that intramuscular injections of suprofen 200 mg/ml are locally and systemically well tolerated even on administration t.i.d. Measurement of the steady-state plasma level was not indicative of altered kinetic behaviour of suprofen. The plasma levels remained constant.(ABSTRACT TRUNCATED AT 250 WORDS)