Effects of environmental variation and timing of births on patterns of nursing and weaning behaviors were examined in Dall's sheep (Ovis dalli dalli) in central Alaska. Indices of plant productivity, growing season, and quality of diet (crude protein in feces) indicated that later plant phenology resulted in a shorter growing season in 1989. Timing of parturition was ca. 2 weeks later and less sychronous in 1989 than in 1988. Females responded to this delay of births, and to forage availability and quality, by nursing lambs for a greater total time following parturition, and by reducing total time spent nursing more rapidly in 1989. Maternal females exhibited behaviors associated with weaning more often during early lactation in 1989, when parturition was delayed. Patterns of maternal care did not correspond with indices of quality of diet, suggesting that these differences did not result from environmental tracking. At far northern latitudes, where the period in which to rear young is limited, females exhibited plasticity in patterns of maternal investment to compensate for variation in environmental conditions and timing of births. This variability in maternal care likely represents an adaptive strategy important even in highly predictable environments with only slight or occasional annual variation.