A series of solenoidal NMR probes were built to measure T1 and T2 relaxation times in vivo in the mouse, over the frequency range of 5 to 60 MHz, using inversion-recovery and spin-echo pulse sequences. KHT tumors growing in the legs of C3H mice were studied and compared with normal mouse legs. The tumor relaxation times were studied at 10 MHz during the course of tumor growth and as a function of frequency when the tumor had a mass of approximately 0.9 g. Mouse legs with tumors have higher T1 and T2 values than those without tumors over the frequency range of 5 to 60 MHz. Significant changes in both relaxation times were detected before a palpable mass could be detected. T1 contrast between normal and tumor-bearing legs decreased with increasing frequency, while T2 contrast remained nearly constant. A comparison between in vivo and in vitro measurements was done using four different types of sample preparation: live mouse, dead mouse, excised whole mouse leg, and tissue sample. These studies showed small but significant differences between the relaxation times measured in vivo and those measured in vitro.