Fluorescence Correlation Spectroscopy (FCS) is a suitable technique to investigate biomol. interactions. The advantage of FCS lies in its wide dynamic range in the time and concn. domain, its sensitivity, and thus in the small amt. of material needed. The characteristic time of the processes that can be measured spans more than eleven orders of magnitude, between several nanoseconds up to hours; the measurable concn. range extends from 0.1 nM to several mM. Here we report on FCS expts. to investigate the properties of the 5-hydroxytryptamine (=serotonin) receptor of type 3 (5HT3-R) in vitro and in vivo. The 5HT3-R is a homopentamer, consisting of subunits of a relative mol. mass of 54 kDa. This neuroreceptor functions as a ligand-gated ion channel, which influences among other things anxiety and depression in human beings. It is therefore a good example for a pharmacol. important membrane receptor. [on SciFinder (R)]