OBJECTIVE To investigate the influence of combined ritonavir (RTV) and saquinavir (soft-gelatin capsule formulation; SQV) on systemic exposure to SQV with a view to optimizing the dosing regimen of combined RTV and SQV antiretroviral therapy. METHODS In this open labelled, randomized, parallel group study, SQV and RTV were administered twice daily for 14 days to groups of eight healthy subjects. The two antiretrovirals were either administered alone (800 mg SQV, regimen A, and 400 mg RTV, B) or in combination at various dose levels (RTV : SQV: 400 : 400 mg, C; 300 : 600 mg, D; 200 : 800 mg, E; 300 : 800 mg, F; 400 : 800 mg, G; and 400 : 600 mg, H). Pharmacokinetic parameters of saquinavir and ritonavir were determined and adverse events, vital signs, and clinical laboratory variables recorded. RESULTS RTV substantially increased the plasma concentration of saquinavir for all dose combinations, compared with SQV alone. Based on the primary statistical analysis there was an overall 17-, 22-, and 23-fold increase in saquinavir AUC(0,24 h) on day 14 with regimens E, F, and G, respectively (with confidence intervals of 10-30, 13-37, and 13-39). The lowest combination dose of RTV (200 : 800 mg; E) significantly increased the saquinavir AUC(0,24 h) from below 5 to 57 microg ml(-1) h, which was higher than the exposure obtained with the 400 : 400 mg twice daily regimen (i.e. 36 microg ml(-1) h). RTV also reduced intersubject variability in AUC(0,24 h) for saquinavir from 105% to 32-68%, and C(max)(0,24 h) from 124% to 30-49%. In contrast, SQV showed no clinically significant effect on the pharmacokinetics of ritonavir. The combination regimens were well tolerated, with the least number of adverse events recorded for the 200 : 800 mg (RTV : SQV) combination regimen. CONCLUSIONS RTV significantly increases saquinavir exposure as a consequence of inhibiting SQV metabolism and possibly P-glycoprotein efflux. Pharmacokinetic and safety profiles obtained in the current study indicate that the use of a combination with a lower dose of RTV and a higher dose of SQV than the 400 : 400 mg combination frequently used in clinical practice should be further explored.