The nuclear spin-lattice relaxation times for the tin isotopes Sn117 and Sn119 have been measured in the temperature range between 20 and 50 mK with a SQUID magnetometer by watching the longitudinal magnetization of the nuclear spins after the application of a rf pulse. The temperature was determined using a SQUID noise thermometer. The Korringa constants were estimated from the τ1 values. The experimental device and the principal methods are described and the results and thermometer applications discussed.