In two experiments detailed observations and quantitative analyses were made of the development and adult expression of grooming in laboratory rats. In the first experiment, 5 litters of rat pups were observed each day from 0-28 days of age, and grooming movements were recorded on paper. Forepaw wipes of the nose appeared by Day 3, followed by eye wipes (Day 6) and ear wipes (Day 8). These movements were integrated into normal-appearing head grooming on Day 11. The mouth was used to groom posterior portions of the body beginning on Day 14 with the belly, and continuing subsequently with hip (Day 15), back (Day 18), and the anogenital region and tail (Day 20). Thus, these aspects of grooming followed a general cephalocaudal progression. In contrast, the development of scratching of the head and body, which began on Day 6, did not follow a systematic order. In the second experiment, 5 adult male rats were observed in glass aquaria, which also served as their living compartments, and bouts of grooming were recorded on videotape for later analysis. Sequential and spatial characteristics of movements were determined by replaying the videotapes in slow motion, while temporal characteristics were determined by a frame-by-frarne analysis of the tapes. This experiment revealed a cephalocaudal progression of acts within bouts of grooming: grooming usually began with the paw-lick- nose-wipe sequence and progressed from there to eye wipes and ear wipes. Mouthing of the torso followed, usually beginning with more anterior portions and terminating with more posterior portions. Scratching with the hindpaws was unpredictably interpolated into the grooming sequence. The experiment also revealed that transitions between grooming different parts of the body were predictable from the spatial and temporal characteristics of the grooming. That is, the last stroke in chains of nose, eye, or ear wipes was reliably slower than previous strokes in the chain, and also tended to be incomplete. Thus, the order in which grooming of specific body areas emerges during early development follows an anterior-posterior progression, and this progression closely parallels the sequence in which body areas are groomed by adult rats. Furthermore, in adults these movements obey specific temporal and spatial rules which can be used to predict transitions of grooming from one body area to the next. This parallel between the ontogenetic and adult expressions of grooming may reflect, respectively, the maturation and activation of genotypically determined, functional units in the central nervous system. In addition, theories now being applied to the problem of transitions between different motive systems (e.g., eating and drinking), may be extended to account for transitions within a motive system (e.g., from grooming of one part of the body to the next).