The color reaction between isonicotinic acid hydrazide (isoniazid, INH) and glutaconic aldehyde liberated from 4-pyridylpyridinium dichloride (I) conforms to Lambert-Beer''s law and is the basis for a sensitive, rapid method for detn. of hydrazides in blood plasma. Plasma proteins are precipitated by adding 4 vols. of 5% trichloroacetic acid. To 2 ml. of protein-free filtrate are added 0.5 ml. 1% aqueous soln. of reagent (I), 0.7 ml. 2 [image]JNaOH, and 0.5 ml. 2 [image]HC1. After dilution to 5 ml. vol. and mixing, the soln. reaches a max. color within 15 min. This color is read in an electrophotometer with a 425-[mu] filter and the amt. of INH is calculated in terms of a standard INH soln. Recovery of 10 fig of INH added to guinea pig plasma or to horse serum was found to vary between 95 and 100%. Blood plasma levels in mice and guinea pigs after oral admn. of 5 and 50 [mu]g. of INH, respectively, were found to reach a max. within 30 min. to 2 hrs. and to fall to low levies within 18 hrs.