InLymnaea egg laying is initiated by the discharge of the neurosecretory caudodorsal cells (CDCs), which release an ovulatory hormone (caudodorsal cell hormone; CDCH) and other peptides. Egg laying is a complex behavioral pattern composed of various postures and action patterns. This paper describes the egg laying behavior of intact and freely behaving animals as it occurs spontaneously after an environmental stimulus (clean water stimulus; CWS), and following injections of highly purified CDCH. CDC discharges were monitored in vivo with chronically implanted electrodes. In these animals the egg laying behavior following spontaneous and electrically elicited CDC discharges was analyzed. 1. The egg laying behavior induced by the CWS consisted of three separate phases that occurred in a fixed temporal sequence. Shortly after transferral to clean water, animals became quiescent (resting). This was followed by an active phase (turning) during which behaviors were performed to prepare the substrate for the subsequent oviposition phase. Following oviposition, animals examined the deposited egg mass. 2. After spontaneous CDC discharges and CDC discharges elicited by selective electrical stimulation of the CDCs (with a chronically implanted fine wire electrode) the egg laying behavior was the same as that following the CWS. All phases of egg laying behavior followed the CDC discharge. 3. In contrast, the egg laying behavior following injections of highly purified CDCH was different to that following the CWS. These animals showed no resting phase. Instead, they continued locomoting and entered directly into the turning phase. The animals that did not lay eggs after injection because they were refractory did not show egg laying behavior. 4. The durations of the joint turning and oviposition phases depended strongly on clutch size, suggesting that the ovulated eggs are instrumental in causing this behavior. 5. We conclude that whenever egg laying is preceded by a CDC discharge, either occurring spontaneously or induced by CWS or electrical stimulation, the behavior differs from that following injections of CDCH directly into the blood. This demonstrates that the presence of CDCH in the blood is not sufficient to produce the full complement of egg laying behaviors and suggests that CDCH, or one of the other peptides released during a CDC discharge, has important local effects within the CNS.