The relative importance that endometrial development and the time of entrance of embryos to the uterine cavity have on their chances of implantation, is controversial. We review pregnancy rate (PR) and implantation rate (IR) on 186 patients selected based on the following criteria: 1) age < or = 35 for patients undergoing ovarian stimulation. 2) transfer of at least two good quality embryos and 3) quality of the transfer: optimum. 131 patients received embryos during a Stimulated Cycle (SC). 67 of them had uterine (UT) and 64 tubal transfers (TT), with PR of 38.8% and 53.1% (N.S.) and IR of 13.9% and 20.9% (p < 0.005) respectively. 55 patients received embryos in a Hormonal Replacement Cycle (HRC). 28 of them had UT and 27 TT. PR were 53.6% and 62.9% (N.S.) and IR 17.0% and 21.6% (N.S.) respectively. Our data suggests that: 1) the delayed entrance of embryos into the uterus and/or their atraumatic transfer into the tubes does not seem to improve their chances of implanting. 2) Endometrial differences between stimulated and hormonal replacement cycles seem not to be of critical importance after 48 to 72 hours after oocyte aspiration. 3) A detrimental effect of the endometrial milieu of stimulated cycles, that express only the first 48 to 72 hours after oocyte aspiration, is suggested.