Mating post partum, concurrent lactation and reproduction in the laboratory rat

32 of 35 sexually mature CFHB rats paired with proven males and thereafter allowed to mate post partum and nurse young concurrent with pregnancy, produced a total of 227 litters. The proportion of females producing litters decreased both with increasing age and parity, and with the number of young nursed. Litter size declined both as age and parity increased, and when gestation was extremely prolonged. The incidence and extent of delayed implantation (gestations greater than 25 days) were affected by the number of young nursed but not by increasing age and parity. There was no evidence for the existence of implantation periods coinciding with multiples of oestrous cycles.