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Perceptual motor development, habituation, and learning in squirrel monkeys were studied under controlled rearing and diet history conditions to determine whether the animal's level of behavioral development was similar to well-nourished animals of his own age (agemates) or his own size (sizemates). From birth to 8 weeks of age, the animals were tested weekly on two items of perceptual motor development, visual locating of a click and visual following of an object. At 18 and 52 weeks of age, they were tested in a standard visual habituation-dishabituation paradigm, and from 44 to 52 weeks, they were tested using a two-choice color discrimination learning procedure. Results indicate that early protein deficiency had a strong retarding effect on early growth, and a significant but less extreme effect on perceptual motor development. Protein deficient animals were retarded compared with agemates but precocious compared with sizemates. The behavioral effects associated with protein deficiency persisted for at least 10 weeks after the diet restriction. (GO)