Rodents identify a mating partner by using olfactory cues. Female rats in estrus spend a longer time sniffing odors of sexually active males when physical contacts are hampered. Sexually active males are attracted by odors of receptive females. The preference in either sex disappears when the subjects are gonadectomized, an effect reversible by supplement with estrogen or testosterone. Thus, circulating sex steroids determine the odor preference in both sexes. In both sexes, olfactory cues get to the basal forebrain; this area includes the preoptic area (POA), substantia innominata and accumbens, structures with estrogen receptor–positive neurons. In estrous females, neurons in the POA are activated during precopulatory motivational behavior. Lesion in this area practically eliminates sexual preference and diminishes the motivation. An increase in locomotion in females in estrus, which apparently depends on estrogen-sensitive POA projections to the midbrain locomotor region, has been considered to embody enhanced sexual motivation.