The endocrine basis of reproductive failure in red fox vixens was examined over two breeding seasons in a total of 11 animals. Weekly blood samples were assayed for progesterone, prolactin, LH and cortisol. Vaginal smears taken every 2 days over the oestrous period indicated that all vixens had mated. Vixens that successfully gave birth to a litter of cubs demonstrated significantly higher plasma progesterone and prolactin concentrations but significantly lower cortisol concentrations than did females that had ovulated, but then failed to whelp. There were no significant differences in plasma LH concentrations. These data suggest that reproductive losses could result from lowered plasma progesterone concentrations, possibly resulting from inadequate luteotrophic support by prolactin. A stress-induced mechanism of reproductive failure is implicated and is discussed in relation to social suppression of reproduction.