Combining an isogravimetric technique and a colorimetric 'on-line' method (Rippe & Stage 1978), filtration capacity (CFC) and diffusion capacity (PS) were simultaneously measured in the maximally vasodilated 'fenestrated' capillary bed of isolated, artificially perfused pancreatic glands in 12 juvenile pigs. Both CFC and PS for Cr-EDTA were about 20 times greater than in the 'continuous' capillary bed of skeletal muscle. With perfusate flow rates of 250 ml/min x 100 g during isogravimetry, PS-Cr-EDTA averaged 110 +/- 10.0 (S.E.) ml/min X 100 g, and diffusion limitation occurred first at flow rates above 300 ml/min X 100 g. CFC was independent of flow rate and averaged 0.641 +/- 0.027 ml/min X 100 g X mmHg. The parallel augmentation of PS-Cr-EDTA and CFC in the fenestrated capillary bed compared with continuous ones seems to reflect both a higher number of capillaries per unit tissue and an increased number of 'small pores' per unit capillary surface, whilst the 'large pore system' appears to be similar. Following bradykinin or histamine infusion, results were similar to those for continuous capillaries (e.g. Rippe, Kamiya & Folkow 1978). Thus, without further vasodilatation CFC increased 3-fold While PS-Cr-EDTA increased only some 25%, and subsequent isoprenaline infusion reversed these effects. Previous studies on continuous capillaries indicate that histamine-type agents act by opening additional 'large pores' in the venular exchange sections (cf. Rippe & Grega (1978, Svensjö 1978), while beta-adrenergic agonists block this effect. The results further suggest that the fenestrae are not involved in these bradykinin-histamine effects, but rather function as a high-density, small pore population.