AbstractA series of solenoidal NMR probes were built to measureT1andT2relaxation timesin vivoin the mouse, over the frequency range of 5 to 60 MHz, using inversionrecovery 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 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 higherT1andT2values 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.T1contrast between normal and tumor‐bearing legs decreased with increasing frequency, whileT2contrast remained nearly constant. A comparison betweenin vivoandin vitromeasurements 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 measuredin vivoand those measuredin vi