Catecholamines (CA) secreted by fetal adreno medullary chromaffin cells (AMCs) play an essential role during the adaptation to extrauterine life. Experiments were performed on primary culture of AMC from E15 and E19 rat fetuses. Animals were humanely killed. The techniques of Ca2+ current, amperometry and capacitance measurements were used. Data are presented as mean +/- SEM. Statistical significance was assessed with unpaired Student’s t-test. At E15, application of digitonin (20 microM) and 10 microM free Ca2+ induce secretion of CA. At both stages, the exocytotic process requires the expression of HVA calcium channels, this current density is not significatively different and its exocytotic efficiency remains the same. The quantal secretion of CA during this prenatal period, was compared by the cumulative integral of amperometric currents induced by depolarisation with 20 s exposure to 70 mM KCl, together with the characteristics of individual spikes. The value of cumulative integrals of amperometric currents at 1 min after KCl application were 3.47 +/- 0.9 pC for E15, (n=8) and 7.24 +/- 0.7 pC for E19 (n=8), p<0.01; such difference is the consequence of the increase in the number of molecules of CA per granule (E15: 164842 +/- 4712, (n= 566); E19: 277067 +/- 9737 (n= 600), p< 0.001). The time delay to release 50% of the total amount of quanta mobilized by KCl was shorter at E19 (n=8) than at E15 (n=8): 5.58 +/- 4.42 s, and 11.49 +/- 16.04 s respectively (p<0.001). During the perinatal period, CA release in AMC is supposed to be controlled by an oxygen sensing mechanism. At E19, hypoxia induces a membrane depolarisation and CA release but at E15, hypoxia induces membrane hyperpolarisation. This dual effect over membrane potential is related to the changing contribution of different types of K+ channels. We compared the effect of glibenclamide (50 microM) over the total K+ current with or without external Ca2+. At E19, IKCa is the main component with 61.19% +/- 7.38 (n = 10) and IKATP: is 14.25 % +/- 2.72, n=10 (p<0.001) of the total IK+ currents. At E15, the predominant K+ current is IKATP with 48.7 % +/- 4.7, n=9 and IKCa is 29.46% +/- 4.94, n = 8, (p<0.001) of the total. At E19, both Apamin (400 nM) and Iberiotoxin (100 nM) induce membrane depolarisation, but hypoxia failed to generate further membrane depolarisation in the presence of apamin. These data show that at E15, the exocytotic process is functional but the quantal size is small compared to E19. The non-neurogenic CA secretion appears when the Apamin-sensitive K+ current is the main component of the total K+ current.