An automated extraction procedure for histamine (I) and fluorometric assay at 360 (excitation)/450 mμ (emission) was used to determine 1-1000 ng I/ml in 30 samples/hr. The sample-to-wash ratio should be 1:2. To decrease the blank fluorescent values in the lower I ranges, both the o-phthalaldehyde (II) and iso-PrOH concentrations progressively decreased from 30 mg I/100 ml 15% iso-PrOH for the 100-1000 ng/ml range to 4 mg I/100 ml 4% iso-PrOH for the 1-10 ng/ml range. A linear response appeared between fluorescence and I concentration over the range 100-1000 ng/ml. Expressed as a relative std deviation the precision of the estimate was 5.4%. Arginine, spermidine, 5-hydroxytryptamine, tryptophan, tubocurarine, adrenaline, isoprenaline, salbutamol, and disodium cromoglycate did not interfere in the assay. Urinary I must be chromatog. separated prior to anal. I values obtained for the lung, blood, mast cells, and rat and guinea pig skin agree with those for bioassay or manual fluorometric assays. The automated method was used to determine I release by antigen from reagin-sensitized human lung and basophils, rat mast cells and skin, and for investigating the in vivo release of I after the administration of I-releasing agents.