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Two experiments were performed to study the effects of bromocriptine and -ergocryptine on oxytocin secretion in lactating rats. In both experiments, after overnight separation from their litters, rats were injected with either vehicle alone or ergot alkaloid plus vehicle; 4 h later the litters were returned.

In the first experiment the mothers were conscious. Treatment did not affect suckling behaviour, number of stretch reactions or litter weight gain in the first 30 min. Oxytocin injection before the second 30 min period of suckling caused no extra milk to be obtained. In the second experiment the mothers were anaesthetized with ethyl carbamate (1·1 g/kg body weight) at the time of the ergot alkaloid or vehicle injection. Changes in intramammary pressure were recorded during suckling. Ergot alkaloids altered neither the number of milk ejections caused by suckling, nor the proportion of milk ejections equivalent to 0·2 mu. or more oxytocin.

In both experiments treatment with ergot alkaloids suppressed secretion of prolactin. It is concluded that (a) in suppressing lactation, bromocriptine and -ergocryptine do not inhibit oxytocin secretion as well as prolactin secretion, and that (b) prolactin secretion is not a necessary concomitant of oxytocin secretion.