Background: We have previously demonstrated that placental chorionic plate arteries (CPAs) at term exhibit a biphasic (transient contraction followed by maintained relaxation) response on exposure to histamine [1]. Here we wished to determine if early pregnancy CPAs displayed similar vascular reactivity. Method: Placentas (N=5) were obtained following elective medical or surgical termination of pregnancy. Gestational age was estimated from date of last menstrual period and confirmed by ultrasound dating. Biopsies were placed into ice-cold HCO₃^–-buffered physiologic salt solution (PSS). Arteries were dissected from the chorionic plate, mounted onto a wire myograph, normalised at 0.9 of L_5.1kPa and equilibrated (37°C; 20 mins; 5%O₂/5%CO₂ ~7% O₂). Contraction was assessed with 120mM potassium solution (KPSS) and the thromboxane-mimetic U46619 (10^-10 to 2×10^-6M). Histamine hydrochloride (HIS; 10^-6M) was added to pre-contracted vessels (EC₈₀ dose of the thromboxane-mimetic U46619 for 30 mins) for 60 mins. Experiments with HIS were also performed in the presence of indomethacin (I; 10^-5M) and indomethacin plus Nω-nitro-L-arginine, (IN; both 10^-5M). Arterial relaxation to donated nitric oxide (NO) was assessed using sodium nitroprusside (SNP; 10^-5M). All data are expressed as median (range). Results: Normalised luminal internal diameters were 486 (210-1008) μm (n=20 arteries). Maximal arterial contraction to KPSS and U46619 was 2.0 (0.4-5.0) kPa and 3.2 (1.2-6.8) kPa (n=20 arteries from N=5 placentas) respectively. HIS induced a biphasic response in pre-contracted CPAs; a transient contraction to 105 (102-118) % of EC₈₀ U46619-induced contraction followed by a maintained relaxation to 95 (23-95) % of EC₈₀ U46619-induced contraction. Contraction and relaxation to HIS alone were not significantly affected by pre-incubation with I or IN (P<0.05; Friedman’s test followed by Dunn’s post hoc test). SNP induced a significant relaxation in pre-contracted CPAs; a maximal relaxation to 37 (2-71) % of EC₈₀ U46619-induced contraction. Conclusion: Our preliminary data suggest that 10^-6M HIS elicits a biphasic response in early pregnancy CPAs; the pattern of response is similar to that seen in term CPAs but the size of the responses are reduced compared to term [1]. The ability to assess placental vascular reactivity in early pregnancy is essential for the development of therapeutic agents to treat pathologies associated with increased placental vascular resistance [2].