Concentrations of plasma LH and FSH were measured by radioimmunoassay in four strains of mice maintained in controlled lighting. Gonadotrophin levels in an unselected control line (Line C) were measured under the following conditions: induced cycles; spontaneous cycles; cycles in absence of males and post-partum cycles. In mice with induced cycles, LH reached a mean of about 40 ng/ml between 16.00 and 17.00 hours of pro-oestrus. Levels of FSH reached a peak of around 2800 ng/ml about 2 hr later, between 19.00 and 20.00 hours, suggesting that the function of FSH is to stimulate growth of the crop of follicles which will ovulate during the succeeding oestrous cycle. Spontaneously cycling mice of Line C also had a mean LH concentration of about 40 ng/ml, but this peak began 1 hr later at 17.00 hours of pro-oestrus and persisted for about 4 hr. No well-defined FSH peak was found. Only two of seventy individually caged females killed during pro-oestrus had LH or FSH levels greater than the mean di-oestrous levels in induced and spontaneously cycling animals. Within 24 hr of parturition, there was no one time when the majority of mice showed elevated levels of LH or FSH. The timing and magnitude of gonadotrophin release during pro-oestrus of induced cycles in lines successfully selected for small litter size, high embryo survival, and high ovulation rate were the same as for Line C, suggesting that the principal effect of selection was probably to alter the sensitivity of the target organs.