The spontaneous acrosomal reaction of mouse spermatozoa during their passage through the male and female genital tract was studied by indirect immunofluorescence staining techniques using antibodies against the outer acrosomal membrane (OAM). Low rates of spontaneously acrosome-reacted spermatozoa could be observed during the passage through the male reproductive tract, namely 4% in the testis (SD +/- 0.8%) and in the caput epididymidis (SD +/- 1.5%), and up to 8% in the cauda epididymidis (SD +/- 0.5%) and in the ductus deferens (SD +/- 0.5%), whereas a higher value of 13% (SD +/- 1%) of reacted spermatozoa was obtained from the corpus epididymidis. Spermatozoa removed from the uterus 20 min up to 14 h post copulation (p.c.) exhibited a constant degree of 8% (SD +/- 6%) acrosomal reaction, whereas 43% (SD +/- 6%) of the oviductal sperms were acrosome-reacted, independent of the time of removal p.c. Rates of spontaneously acrosome-reacted spermatozoa from the male and female reproductive tract could be increased by 2-fold and up to 7-fold by an in vitro incubation in modified Tyrode's medium for 210 min. On the basis of our results wer postulate a basic level of spontaneously acrosome-reacted spermatozoa in the oviduct at ovulation time, which might be relevant to ensure the environmental conditions for the fertilization process. Additionally, we offer a possibility of optimizing in vitro fertilization techniques.