Penetration of cefpiramide and cefazolin into a specific extravascular fluid was measured with rabbits bearing capsules in the peritoneal cavity. A general feature of slow accumulation and elimination of drugs from extravascular sites having low surface area/volume ratios has also been observed in this study. The capsular concentration-time profiles were well expressed by the following equation: C(CF) = A(CF)[e-kel(CF)(t-to)-e-kp(CF)(t-to)], where C(CF), A(CF), kp(CF), kel(CF), and to indicate capsular concentration at time t, constant for the dimension of concentration, capsule penetration rate constant, capsule elimination rate constant, and lag time before penetration occurs, respectively. The kp(CF), kel(CF), and to were 0.139 h-1, 0.059 h-1, and 0.45 h, respectively, for cefpiramide, and 0.448 h-1, 0.0145 h-1, and 0.14 h, respectively, for cefazolin. A(CF) was 22.7 micrograms/ml for cefpiramide and 4.53 micrograms/ml for cefazolin, being parallel to the area under the plasma concentration-time curve for free drug from to to infinity (20.1 micrograms.h/ml for cefpiramide and 3.43 micrograms.h/ml for cefazolin). In conclusion, it is suggested that as well as kp(CF) and kel(CF), the area under the plasma concentration-time curve for free drug from to to infinity may play an important role regarding the circulating reservoir of drugs in determining capsular concentration-time profiles in experimental models for particular extravascular sites of infection, like abscesses into which drugs cannot easily penetrate.