The influence of perfusion medium on in vitro recovery of 14C-oleate to microdialysis probes was investigated. Four perfusion media were investigated: water, 4% bovine serum albumin (BSA) in Ringer solution, 2.25% glycerol in water and a 20% soybean oil/egg phospholipid emulsion (Intralipid). The following order of recovery was found (highest recovery first): BSA = Intralipid greater than glycerol greater than water. The recoveries at 0.5 microliter/min. perfusion rate of the dialysis probe were 4.8%, 4.4%, 2.6% and 1.2% respectively. It was attempted to measure 14C-oleate after intravenous infusion. The samples were extracted with hexane/isopropyl alcohol in order to separate the free fatty acid from products of oxidation which are hydrophilic. Dialysis probes were implanted in interscapular adipose tissue, epididymal fat, muscle, liver and jugular vein of rats. We were not able to detect unbound fatty acids, neither basally nor after stimulation with 1 mg/kg norepinephrine intraperitoneally. However, products of oxidation from the infused oleate were released in response to norepinephrine stimulation. It is concluded that the recovery of lipophilic compounds in microdialysis devices can be improved by means of a lipophilic perfusion medium or by means of e.g. BSA to which the compound binds. The free fatty acid levels were too small to be measureable in vivo in the present study.