The problem of histamine metabolism has long been the subject of research. The excretion of histamine during pregnancy has been studied in cats, mice, hamsters, and many other animals, and also in human beings. However, few papers have been published on the levels of histamine in the blood of pregnant animals. In this paper the variation in blood histamine levels in pregnant mares from the first month of pregnancy until term is reported. Materials and MethodsSix pregnant mares of the PSI strain plus two controls were studied. The method followed was that of G-addum modified by Code (1937). Blood was collected in syringes containing heparin and put into· flasks also containing heparin. Proteins· were precipitated with 10% (w/v) trichloroacetic acid and, after standing for 1-1 hr, filtered by suction. This was repeated three times. To the filtrate was added concentrated HOI and this solution was boiled for 90 min in a reflux air condenser, water being added to prevent dessication. Then twice-distilled ethanol was added and the residue dried in vaouo on a hot water-bath. This dried residue was suspended in distilled water and filtered. This was repeated twice. The filtrate was neutralized with 0·2N NaOH and tested on a guinea-pig intestine against a standard histamine solution (1 mgtl ml Tyrode's solution). ResultsFigure 1 shows a typical assay of an extract of pregnant mare's blood. Further details of the assay are given in the legend to the figure. Figure 2 shows the variations in blood histamine levels during pregnancy. It is seen that there is already an increase in histamine concentration in the first two months of pregnancy which is significantly different from control values at P = 0·02. There is a sharp decrease (to control levels) in the third and fourth months, and a slight (not significant) rise in the fifth and sixth months. However, there is a significant increase in blood histamine level In the seventh and eighth months (P < 0·01) and also in the ninth and tenth months of pregnancy (P = 0·05). DiscussionThese results agree with those reported by Kahlson and Rosengren (1968) for other species, namely that there is an increase in histamine concentration in· the blood in the last third or last quarter of pregnancy. However, as we do not have data on histamine levels in mare's blood immediately after birth, we are unable to compare such levels with those of other species.• Manuscript received 21 January 1972. t SecQ8.0 de DoenQas Carenciais e Metab6licas,