The CD (Charles River) strain of rats differs in several important respects from our inbred Osborne Mendel (O-M) strain. The critical period for the ovulatory surge of gonadotropin in proestrus is less sharply limited in CD rats. They are much more refractory to preoptic (POA) electrochemical stimulation for ovulation, not only in normally cycling females, but strikingly so in androgen-sterilized (TP) females, as well as in rats having spontaneous persistent estrus (PE). In proestrous 4-day cyclic CD females ‘blocked’ with pentobarbital, a much larger stimulus (100 µA × 30 sec = 3000 µcoulombs) was required to fully ovulate all subjects than was needed in similar O-M rats (10 µA × 20 sec = 200 µcoulombs). While 5/6 O-M PE rats ovulated in response to massive unilateral POA stimulation (3000 µcoulombs), all of 6 CD PE rats failed to ovulate. Even bilateral POA stimulation with these parameters induced ovulation in only 3/10 rats. O-M TP rats had low POA thresholds close to those of cyclic O-M females, while only 1/6 CD TP rats gave even a partial response (5 tubal ova) to massive bilateral stimulation. CD TP rats also differed from O-M TP rats by secreting abundant prolactin in spite of the constant estrus. This was shown by prominent milk cysts in the mammary glands at 8–9 months of age and by spontaneous pseudopregnancy after corpora lutea were induced by LH injection. O-M TP rats had no mammary development and experienced only a short diestrus after LH injection. Whether there is a common basis for these strain differences remains uncertain.