Four, three- to four-week-old Brahman calves, with a common sire and a common maternal grandsire, developed progressive muscular weakness which was exacerbated by exercise and improved with rest. Although the calves remained alert and continued sucking, their neuromuscular condition deteriorated and they had to be euthanased within four to six weeks. The one calf in which an attempt was made to diagnose the condition responded quickly, but transiently, to the intravenous administration of edrophonium chloride and for a longer period to oral pyridostigmine bromide. Repeated stimulation of a peripheral nerve resulted in a decrease in the amplitude of the evoked muscle action potential. A definitive diagnosis could not be made but the condition showed many similarities to congenital myasthenia gravis, which has not previously been described in cattle.