A gentamicin radioimmunoassay is described for microliter and nanoliter samples. The assay was performed in the inner ears’ fluids (perilymph and endolymph) of rats. Endolymph and perilymph samples were collected under oil with a glass capillary micropipette. The absolute threshold of sensitivity was 20 · 10––12 g. Considering the maximum sampling volume, 15–400 · 10––9 and 10 · 16––6 l for endolymph and perilymph respectively, the limit of detection is 4.19 · 10––4μM · ml––1 in endolymph, 1.26 · 10––4μM · ml––1 in perilymph. Clinical a