In a single dose cross-over study with 12 healthy male volunteers the plasma concentrations of diclofenac (CAS 15307-86-5) and codeine (CAS 76-57-3) were determined after oral and rectal application of formulations containing 50 mg of each drug. For kinetic analysis of the concentration-time profiles non-compartmental as well as compartmental procedures were used. The compartment model included two disposition compartments supplemented by a dissolution and drug absorption step. For both compounds, the AUC0-infinity values of the two treatments were similar with only a slightly higher AUC for the suppositories, which was not found to be significantly different under the employed conditions (p > 0.05). Referring to the pharmacokinetic parameters Cmax and tmax typical differences between oral and rectal formulations were observed. For the suppositories, diclofenac and codeine average peak plasma concentrations were only half as high as for the tablets, whereas the respective tmax values were doubled. The results obtained show a similar extent of diclofenac and codeine bioavailability for both administration routes, but the rate of drug input was lower for the suppositories. The total mean input time (MITtot) was found to be significantly longer for the suppositories. This seems to be caused by a slow release from the dosage form or dissolution of the drugs, which is confirmed by a longer MITtot compared to the MRTsys in most of the volunteers.