The risks of conception, due to sexual intercourse at various times before and after the periovulatory rise in the woman's basal body temperature, are evaluated. In general, the risk is small nine or more days before, and two or more days after, the first day of elevated temperature. The model for the conception probabilities assumes exponential survival distributions for sperm and egg and a specific probability that an egg is viable. Median survival times for sperm and egg are estimated at 1.0 days and 0.5 days, respectively. The Pearl indices for intercourse on the third and fourth day of elevated temperatures are 4.7 and 1.2 pregnancies per 100 woman-years, respectively.