Three sources of harmful health effects from nuclear magnetic resonance (NMR) in vivo techniques have been examined with the following conclusions: (a) Static magnetic fields. Harmful effects on humans and reproducible cellular, biochemical, or genetic effects have not yet been observed at fields less than 2 Tesla (20,000 gauss), (b) Changing magnetic fields. The threshold for effects of induced currents is above that produced from < 1 to 100 Hz sinusoidal field changes with a maximum field of 5 mT (50 gauss). Waveform, repetition rate, maximum B field, and duration of exposure are parameters requiring further study, (c) Radiofrequency (RF) heating. A practical upper level for absorbed power is 4 W/kg in medically important studies of short duration (less than 10 min). For long-term studies, 1.5 W/kg is a reasonable level in low humidity environments. The power absorbed by the subject can be estimated by measuring the RF coil Q before and after the subject is placed in the NMR instrument. Large metal objects will absorb power in proportion to the conductivity of the device or prosthesis.