This theoretical paper sets the stage for subsequent experimental reports on mobility and immobility in the Arkansas Line of Nervous Pointer dogs as contrasted with kennel mates of the normal line. Exactly opposite to the normal animals, the nervous dogs show marked inhibition of heart rate and musculoskeletal responses to man, including reduced ambulation and durable immobility following inversion and brief manual restraint in an open sling. The sling immobility of the nervous dogs (which may not differ basically from their freezing in upright position) might be designated as "tonic immobility" (TI). We hypothesize that such immobility, common in laboratory and natural conditions in many species, should be divided into two classes: hypotonic (cataleptic) and hypertonic (catatonic). We provide examples of such behaviors, particularly in man and dog, and cursorily review TI studies of other species. Neurophysiologic and neurochemical studies which bear on possible immobility mechanisms are briefly noted. We suggest that inconsistencies in reported concommitants of TI might result from failure to discriminate between types of behavioral responses.