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Removing milker units as soon as milk flow stopped was compared to milking for a fixed time of 12 min in an 8-wk trial with 60 cows. Teats were dipped in broth culture of approximately 10(9) colony forming units per ml of Staphylococcus aureus, Streptococcus agalactiae, and Streptococcus uberis for 4 wk. More new infections of quarters occurred in the 12-min group. In trials 2 and 3, 20 additional cows were milked with automatic milking machine detachers and 20 cows were milked with a flow control unit for 12 min each trial. New infection rates were highest in the two groups milked for 12 min. Age for lactation and cows were associated with most of the variation in the new infection rate. Cows with pointed teat ends had the highest rate of new infection. Effects of fixed-time milking on increased mammary infection and clinical mastitis were small.

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In spite of the importance of prolactin and adrenal corticosteroids in the normal control of the mammary gland (Cowie & Tindal, 1971) there has been no report of their simultaneous measurement in rat plasma during pregnancy and lactation. There is no agreement on the pattern of change in corticosteroid concentration at the time of lactogenesis and parturition (Gala & Westphal, 1965;Kuhn, 1969).Primiparous CFE strain rats, allotted randomly to groups on day 0 of pregnancy, were decapitated without prior disturbance within 1 min after removal from their cage, between 08.30 and 09.30 h. Plasma was stored at \m=-\20\s=deg\C,total body weight after bleeding and adrenal weight were recorded. The day on which a vaginal plug was found and the day of parturition were designated day 0 of pregnancy and lactation respectively. Litter size was adjusted to six on day 0 of lactation.Plasma prolactin concentration was assayed in triplicate for each rat by the National Institute of Arthritis and Metabolic Diseases (NIAMD) rat-prolactin radioimmunoassay (Neill & Reichert, 1971) against the standard rat prolactin preparation (NIAMD RP-1 30i.u./mg). Corticosteroid concentration was measured in 0-01 ml duplicate samples of plasma per rat by the competitive protein-binding radioassay of Murphy (1967). [l,2-3H]Corticosterone (sp. act. 49-5 Ci/mmol, New England Nuclear Corporation), and corticosterone (4-pregnen-ll/?,21-diol-3,20-dione, Mann Research Laboratories) were used as the tracer and standard respectively. Plasma prolactin concentration decreased from 114-4 + 19-4 (s.e.m.) ng/ml on day 0 of pregnancy to a low of 21-0 + 1-4 ng/ml on day 10 of pregnancy, then increased to approximately 33 ng/ml on days 15 and 18 of pregnancy, and reached a peak of 133-0 + 34-5 ng/ml on day 5 of lactation. The concentration of prolactin gradually declined to 46-1 + 9-9 ng/ml on day 20 of lactation (Fig. 1). This is in agreement with the work of Amenomori, Chen & Meites (1970). Increased within-group variation during lactation was probably due to the effect of suckling, since studies in our laboratory as well as in others have shown that suckling stimulates the release of prolactin.Plasma corticosteroid concentration showed a high degree of within-group varia¬ tion with no significant differences between groups throughout pregnancy and % Present address:

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