The hypothesis that an increase in serum luteinizing hormone (LH) concentration occurs before the onset of puberty in the gilt was tested. Blood samples were collected from 14 gilts, 166 d of age and 72.6 +/- 1.8 kg body weight, every 15 min for 8 h 1 d after jugular cannulation. Gilts were checked daily for estrus with a boar. The sampling procedure was repeated 21 and 33 d later, by which time three gilts had attained puberty (first estrus with continued regular estrous cycles), and then every 7 d thereafter until an additional five gilts attained puberty. Serum LH and estrogen (E) concentrations were quantitated by radioimmunoassay. Bleeding periods were grouped into seven common physiological ages before puberty; age 1 greater than or equal to 57 d (n = 4), age 2 = 43 to 50 d (n = 5), age 3 = 31 to 38 d (n = 4), age 4 = 22 to 26 d (n = 5), age 5 = 17 to 19 d (n = 4), age 6 = 10 to 12 d (n = 5) and age 7 = 3 to 5 d (n = 5). Mean serum LH concentration (ng/ml, mean +/- SE) at age 7 (1.0 +/- .1) was greater (P less than .01) than at ages 2, 3, 4 and 5 (6 +/- .1) but not ages 1 and 6 (.8 +/- .1).(ABSTRACT TRUNCATED AT 250 WORDS)