A nursing bottle assembly having a container with a closed bottom end, a top end having an opening therein for receiving liquid into an interior of the container, a rim at the top end of the container also with an inner diameter defining the size of the opening at the top end of the container, a threaded collar, a nipple, and a vent insert configured and arranged to provide closure for the opening at the top end of the container and to further provide venting of the interior space of the container during use of said bottle assembly, the vent insert having a lateral vent locating across the diameter of said vent insert, opening on both ends towards said collar and having a centred hole towards the container, said vent insert sealing to said container upon said rim and beneath said collar, a reservoir, said vent insert sealing to said reservoir and communicating with the interior space of said container, said vent insert having an internal vent tube with a distal end disposed generally within the interior space of the reservoir, the lower end of the internal vent tube being open to the interior space of the container, wherein the improvement comprises: said reservoir having a hollow cylindrical shape with an open top and an opposite smaller bottom end; the vent insert having a vent tube connecting to the reservoir and having a hollow contoured shape, a larger diameter of the contoured shape locating proximal to said reservoir; and an opposite narrow diameter distally from said reservoir and being disposed near the bottom of the container, said vent tube having a narrowing taper throughout its length of said vent tube with a distal end disposed within the interior space of the container, wherein said larger diameter of said vent tube is no more than the diameter of said reservoir and exceeds said narrow diameter of said vent tube, and being centred upon said internal vent, said internal vent tube having a hollow cylindrical form, centred upon said vent insert, perpendicular to said lateral vent, and in communication with said reservoir, said vent insert having a major seal for sealing between said vent insert and said container and a minor seal for sealing between said vent insert and said reservoir, said vent insert having a hollow cylindrical shape open on both ends with a perimeter wall; said major seal depending from said wall towards said container and having a circumferential bulge for a frictional engagement of said vent insert with said container; said minor seal concentric with said major seal; and having a circumferential bulge for a frictional engagement of said vent insert with said reservoir; an internal vent tube having a hollow cylindrical form coaxial with said minor seal, centred upon said vent insert, perpendicular to said lateral vent, and in communication with said reservoir; and whereby when pressure is applied upon said container, the feeding liquid within said container moves within said vent tube towards said larger diameter of said vent tube causing the pressure to decrease within said vent tube, and thus preventing leaks from the bottle. said reservoir having a hollow cylindrical shape with an open top and an opposite smaller bottom end; the vent insert having a vent tube connecting to the reservoir and having a hollow contoured shape, a larger diameter of the contoured shape locating proximal to said reservoir; and an opposite narrow diameter distally from said reservoir and being disposed near the bottom of the container, said vent tube having a narrowing taper throughout its length of said vent tube with a distal end disposed within the interior space of the container, wherein said larger diameter of said vent tube is no more than the diameter of said reservoir and exceeds said narrow diameter of said vent tube, and being centred upon said internal vent, said internal vent tube having a hollow cylindrical form, centred upon said vent insert, perpendicular to said lateral vent, and in communication with said reservoir, said vent insert having a major seal for sealing between said vent insert and said container and a minor seal for sealing between said vent insert and said reservoir, said vent insert having a hollow cylindrical shape open on both ends with a perimeter wall; said major seal depending from said wall towards said container and having a circumferential bulge for a frictional engagement of said vent insert with said container; said minor seal concentric with said major seal; and having a circumferential bulge for a frictional engagement of said vent insert with said reservoir; an internal vent tube having a hollow cylindrical form coaxial with said minor seal, centred upon said vent insert, perpendicular to said lateral vent, and in communication with said reservoir; and whereby when pressure is applied upon said container, the feeding liquid within said container moves within said vent tube towards said larger diameter of said vent tube causing the pressure to decrease within said vent tube, and thus preventing leaks from the bottle.